Summary of Significant Accounting Policies Summary of Significant Accounting Policies - New Accounting Standards and Interpretations (Details) (USD $)	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Accounting Policies [Abstract]
MBS securities recorded upon adoption of revised accounting standard	$ 1,913,000,000
CRT securities recorded upon adoption of revised accounting standard	4,600,000
Repurchase agreements recorded upon adoption of revised accounting standard for repurchase agreement financing	1,519,593,000	0
Cumulative effect adjustment on adoption of revised accounting standard for repurchase agreement financing	$ 4,537,000	$ 0