Fair Value of Financial Instruments - Valuation of MBS Investments (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012 Agreements	Dec. 31, 2011 Agreements
Agency MBS [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unrealized loss position, less than on year, Fair Value	$ 271,727	$ 680,101
Unrealized loss position, less than on year, Unrealized Loss	(1,655)	(6,765)
Number of Securities held, less than 12 months	33	54
Unrealized loss position, Twelve months or longer, Fair Value	379,030	160,544
Unrealized loss position, 12 months or longer, Unrealized Losses	(5,580)	(1,684)
Number of securities held, 12 months or longer	37	27
Number of Securities in Continuous Unrealized Loss position	330	266
Non-Agency MBS [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unrealized loss position, less than on year, Fair Value	20,369	153,974
Unrealized loss position, less than on year, Unrealized Loss	(1,338)	(5,075)
Number of Securities held, less than 12 months	7	6
Unrealized loss position, Twelve months or longer, Fair Value	8,198	2,993
Unrealized loss position, 12 months or longer, Unrealized Losses	(801)	(379)
Number of securities held, 12 months or longer	8	7
Number of Securities in Continuous Unrealized Loss position	59	44
Collateralized Mortgage Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Continuous Unrealized Loss Position, Fair Value	679,324	997,612
Continuous Unrealized Loss Position, Unrealized Losses	$ (9,374)	$ (13,903)
Number of Securities in Continuous Unrealized Loss position	85	94