Quarterly report pursuant to Section 13 or 15(d)

Fair Value of Financial Instruments (Details)

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Fair Value of Financial Instruments (Details) - USD ($)
$ in Thousands
Mar. 31, 2018
Dec. 31, 2017
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Repurchase agreements $ 19,148,679 $ 19,451,207
Long-term Federal Home Loan Bank Advances 865,024  
Maturity Over One Year [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Repurchase agreements $ 250,000 $ 112,500