Quarterly report pursuant to Section 13 or 15(d)

Derivative Instruments and Hedging Activities Schedule of Interest Rate Swaptions (Details)

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Derivative Instruments and Hedging Activities Schedule of Interest Rate Swaptions (Details) - USD ($)
3 Months Ended 12 Months Ended
Mar. 31, 2018
Dec. 31, 2017
Mar. 31, 2017
Dec. 31, 2016
Derivative [Line Items]        
Notional $ (18,429,288,000) $ (31,226,878,000) $ (15,935,535,000) $ (18,802,500,000)
Interest Rate Swaption [Member] | Long [Member] | Variable Income Interest Rate [Member] | Less Than Six Months Remaining Maturity [Member]        
Derivative [Line Items]        
Cost Basis (8,461,000) (21,380,000)    
Fair Value $ 10,754,000 $ 17,736,000    
Weighted Average Remaining Maturity 3 months 20 days 4 months 1 day    
Interest Rate Swaption [Member] | Long [Member] | Fixed Income Interest Rate [Member] | Less Than Six Months Remaining Maturity [Member]        
Derivative [Line Items]        
Cost Basis   $ (4,660,000)    
Fair Value   $ 2,982,000    
Weighted Average Remaining Maturity   3 months 22 days    
Interest Rate Swaption [Member] | Short [Member] | Variable Income Interest Rate [Member] | Less Than Six Months Remaining Maturity [Member]        
Derivative [Line Items]        
Cost Basis   $ (7,950,000)    
Fair Value   $ 5,619,000    
Weighted Average Remaining Maturity   4 months 20 days    
Interest Rate Swaption [Member] | Short [Member] | Variable Income Interest Rate [Member] | Six Months or Longer Remaining Maturity [Member]        
Derivative [Line Items]        
Cost Basis $ (6,808,000)      
Fair Value $ (6,783,000)      
Weighted Average Remaining Maturity 11 months 6 days      
Interest Rate Swaption [Member] | Short [Member] | Fixed Income Interest Rate [Member] | Less Than Six Months Remaining Maturity [Member]        
Derivative [Line Items]        
Cost Basis $ (23,580,000) $ (16,260,000)    
Fair Value $ (9,301,000) $ 4,694,000    
Weighted Average Remaining Maturity 2 months 15 days 5 months 5 days    
Interest Rate Swaption [Member] | Short [Member] | Fixed Income Interest Rate [Member] | Six Months or Longer Remaining Maturity [Member]        
Derivative [Line Items]        
Cost Basis $ (6,962,000)      
Fair Value $ (6,501,000)      
Weighted Average Remaining Maturity 11 months 6 days      
Underlying Swap [Member] | Long [Member] | Variable Income Interest Rate [Member] | Less Than Six Months Remaining Maturity [Member]        
Derivative [Line Items]        
Weighted Average Remaining Maturity 5 years 3 years 9 months    
Notional $ (4,095,000,000) $ (7,200,000,000)    
Underlying Swap [Member] | Long [Member] | Variable Income Interest Rate [Member] | Less Than Six Months Remaining Maturity [Member] | London Interbank Offered Rate (LIBOR) [Member]        
Derivative [Line Items]        
Weighted Average Fixed Interest Rate 2.98% 2.27%    
Underlying Swap [Member] | Long [Member] | Fixed Income Interest Rate [Member] | Less Than Six Months Remaining Maturity [Member]        
Derivative [Line Items]        
Weighted Average Remaining Maturity   10 years    
Notional   $ (2,300,000,000)    
Weighted Average Fixed Interest Rate   2.095%    
Underlying Swap [Member] | Short [Member] | Variable Income Interest Rate [Member] | Less Than Six Months Remaining Maturity [Member]        
Derivative [Line Items]        
Weighted Average Remaining Maturity   10 years    
Notional   $ (1,693,000,000)    
Underlying Swap [Member] | Short [Member] | Variable Income Interest Rate [Member] | Less Than Six Months Remaining Maturity [Member] | London Interbank Offered Rate (LIBOR) [Member]        
Derivative [Line Items]        
Weighted Average Fixed Interest Rate   2.70%    
Underlying Swap [Member] | Short [Member] | Variable Income Interest Rate [Member] | Six Months or Longer Remaining Maturity [Member]        
Derivative [Line Items]        
Weighted Average Remaining Maturity 10 years      
Notional $ (280,000,000)      
Underlying Swap [Member] | Short [Member] | Variable Income Interest Rate [Member] | Six Months or Longer Remaining Maturity [Member] | London Interbank Offered Rate (LIBOR) [Member]        
Derivative [Line Items]        
Weighted Average Fixed Interest Rate 2.99%      
Underlying Swap [Member] | Short [Member] | Fixed Income Interest Rate [Member] | Less Than Six Months Remaining Maturity [Member]        
Derivative [Line Items]        
Weighted Average Remaining Maturity 5 years 5 years 7 months 9 days    
Notional $ (9,710,000,000) $ (5,141,000,000)    
Underlying Swap [Member] | Short [Member] | Fixed Income Interest Rate [Member] | Less Than Six Months Remaining Maturity [Member] | London Interbank Offered Rate (LIBOR) [Member]        
Derivative [Line Items]        
Weighted Average Fixed Interest Rate 2.32% 1.89%    
Underlying Swap [Member] | Short [Member] | Fixed Income Interest Rate [Member] | Six Months or Longer Remaining Maturity [Member]        
Derivative [Line Items]        
Weighted Average Remaining Maturity 10 years      
Notional $ (280,000,000)      
Underlying Swap [Member] | Short [Member] | Fixed Income Interest Rate [Member] | Six Months or Longer Remaining Maturity [Member] | London Interbank Offered Rate (LIBOR) [Member]        
Derivative [Line Items]        
Weighted Average Fixed Interest Rate 2.99%