Fair Value of Financial Instruments - Carrying Values and Fair Values of Financial Assets and Liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Financial Liabilities:
Payable for unsettled purchases, aggregate cost	$ 14,936
Quoted Prices in Active Markets for Identical Assets (Level 1)
Financial Assets:
Cash	243,294	$ 289,925
Significant Observable Inputs (Level 2)
Financial Assets:
Cash collateral posted to counterparties	306,104	263,799
Principal payments receivable		37
Accrued interest receivable	28,883	34,500
Financial Liabilities:
Repurchase Agreements	9,923,859	11,570,481
Payable for unsettled purchases, aggregate cost	14,955
Accrued interest payable- repurchase agreements	6,255	7,724
Carrying Value
Financial Assets:
Cash	243,294	289,925
Cash collateral posted to counterparties	306,104	263,799
Principal payments receivable		37
Accrued interest receivable	28,883	34,500
Financial Liabilities:
Repurchase Agreements	9,923,859	11,570,481
Payable for unsettled purchases, aggregate cost	14,955
Accrued interest payable- repurchase agreements	6,255	7,724
Fair Value
Financial Assets:
Cash	243,294	289,925
Cash collateral posted to counterparties	306,104	263,799
Principal payments receivable		37
Accrued interest receivable	28,883	34,500
Financial Liabilities:
Repurchase Agreements	9,923,859	11,570,481
Payable for unsettled purchases, aggregate cost	14,955
Accrued interest payable- repurchase agreements	$ 6,255	$ 7,724