Annual report pursuant to Section 13 and 15(d)

Derivative Instruments and Hedging Activities Schedule of Interest Rate Swap Payers Associated with the Investment Portfolio (Details)

v3.6.0.2
Derivative Instruments and Hedging Activities Schedule of Interest Rate Swap Payers Associated with the Investment Portfolio (Details)
$ in Thousands
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
Dec. 31, 2014
USD ($)
Derivative [Line Items]      
Derivative, Notional Amount $ 18,802,500 $ 21,998,381 $ 48,115,523
Interest Rate Swap [Member]      
Derivative [Line Items]      
Derivative, Notional Amount 20,371,063 14,268,806 $ 18,584,000
Interest Rate Swap [Member] | Interest Rate Risk Associated with the Investment Portfolio [Member] | Long [Member]      
Derivative [Line Items]      
Derivative, Notional Amount $ 5,250,000 $ 3,250,000  
Derivative, Average Fixed Interest Rate 1.504% 1.787%  
Derivative, Average Variable Interest Rate 0.972% 0.503%  
Derivative, Average Remaining Period Until Maturity 2.16 3.74  
Interest Rate Swap [Member] | Interest Rate Risk Associated with the Investment Portfolio [Member] | Derivative Maturity Over One And Within Two Years From Balance Sheet Date [Member] | Long [Member]      
Derivative [Line Items]      
Derivative, Notional Amount $ 4,040,000    
Derivative, Average Fixed Interest Rate 1.307%    
Derivative, Average Variable Interest Rate 0.973%    
Derivative, Average Remaining Period Until Maturity 1.58    
Interest Rate Swap [Member] | Interest Rate Risk Associated with the Investment Portfolio [Member] | Derivative Maturity Over Two And Within Three Years From Balance Sheet Date [Member] | Long [Member]      
Derivative [Line Items]      
Derivative, Notional Amount   $ 2,040,000  
Derivative, Average Fixed Interest Rate   1.563%  
Derivative, Average Variable Interest Rate   0.487%  
Derivative, Average Remaining Period Until Maturity   2.94  
Interest Rate Swap [Member] | Interest Rate Risk Associated with the Investment Portfolio [Member] | Derivative Maturity Over Three And Within Four Years From Balance Sheet Date [Member] | Long [Member]      
Derivative [Line Items]      
Derivative, Notional Amount $ 310,000    
Derivative, Average Fixed Interest Rate 1.545%    
Derivative, Average Variable Interest Rate 0.881%    
Derivative, Average Remaining Period Until Maturity 3.59    
Interest Rate Swap [Member] | Interest Rate Risk Associated with the Investment Portfolio [Member] | Derivative Maturity Over Four Years From Balance Sheet Date [Member] | Long [Member]      
Derivative [Line Items]      
Derivative, Notional Amount $ 900,000 $ 1,210,000  
Derivative, Average Fixed Interest Rate 2.378% 2.164%  
Derivative, Average Variable Interest Rate 0.997% 0.531%  
Derivative, Average Remaining Period Until Maturity 4.24 5.08