Repurchase Agreements - Narrative (Details) - USD ($)	Sep. 30, 2015	Dec. 31, 2014
Disclosure of Repurchase Agreements [Abstract]
Repurchase agreements	$ 1,943,299,000	$ 1,436,651,000
Fair Value of securities pledged, including accrued interest receivable	2,066,450,000	1,522,905,000
Cash Held on Deposit with Repurchase Agreement Counterparties	$ 2,046,000	$ 2,616,000
Weighted Average Borrowing Rate	0.47%	0.36%
Maximum Amount at Risk	$ 122,700,000
Pledged Unsettled Securities Sold	$ 84,100,000