Repurchase Agreements - Narrative (Details) - USD ($)	Jun. 30, 2015	Dec. 31, 2014
Disclosure of Repurchase Agreements [Abstract]
Repurchase agreements	$ 2,014,071,000	$ 1,436,651,000
Fair Value of securities pledged, including accrued interest receivable	2,123,621,000	1,522,905,000
Cash Held on Deposit with Repurchase Agreement Counterparties	$ 14,397,000	$ 2,616,000
Weighted Average Borrowing Rate	0.39%	0.36%
Maximum Amount at Risk	$ 122,900,000