Debt Securities (Details) (USD $)	6 Months Ended				6 Months Ended	12 Months Ended	6 Months Ended	12 Months Ended	6 Months Ended	12 Months Ended	6 Months Ended	12 Months Ended	6 Months Ended	12 Months Ended
	Jun. 30, 2012	Apr. 16, 2012	Mar. 31, 2012	Dec. 31, 2011	Jun. 30, 2012 Mortgage-backed securities	Dec. 31, 2011 Mortgage-backed securities	Jun. 30, 2012 CMBS Y	Dec. 31, 2011 CMBS Y	Jun. 30, 2012 CMBS Minimum	Dec. 31, 2011 CMBS Minimum	Jun. 30, 2012 CMBS Maximum	Dec. 31, 2011 CMBS Maximum	Jun. 30, 2012 RMBS Y	Dec. 31, 2011 RMBS Y
Mortgage-Backed Securities Available-for-Sale
Purchased Amortized Cost					$ 899,161,000	$ 347,777,000	$ 662,117,000	$ 177,353,000					$ 237,044,000	$ 170,424,000
Credit OTTI					(7,909,000)	(6,001,000)							(7,909,000)	(6,001,000)
Recorded Amortized Cost					891,252,000	341,776,000	662,117,000	177,353,000					229,135,000	164,423,000
Non-Credit OTTI					(1,354,000)	(1,310,000)							(1,354,000)	(1,310,000)
Unrealized Gains					21,377,000	3,367,000	12,932,000						8,445,000	3,367,000
Unrealized Losses					(2,770,000)	(2,099,000)		(567,000)					(2,770,000)	(1,532,000)
Net Fair Value Adjustment					17,253,000	(42,000)	12,932,000	(567,000)					4,321,000	525,000
Fair Value	908,505,000			341,734,000	908,505,000	341,734,000	675,049,000	176,786,000					233,456,000	164,948,000
Weighted Average Coupon (as a percent)							3.80%	2.10%					2.00%	1.00%
Weighted Average Life (WAL) (in years)							3.6	3.5					3.9	4.8
Weighted Average Yield (as a percent)							6.70%						9.60%
Description of variable rate basis					one-month LIBOR	one-month LIBOR	LIBOR	LIBOR					LIBOR	LIBOR
Effective variable rate basis (as a percent)					0.24575%	0.2953%
Fair value of securities not rated							578,500,000
Investment position rated BB+							97,000,000
Loan-to-value ratio of securities not rated (as a percent)									39.00%	39.00%	44.00%	44.00%
Aggregate participating investment in senior notes	578,500,000
Aggregate participating investment in senior loans			115,500,000
Remaining participating investment in senior loans certificated		115,500,000
Aggregate participating investment in securities			387,600,000
Face value of participating investment in senior loans	$ 608,900,000
Face value of investment to aggregate investment (as a percent)	8.20%
Carrying value of investment to total assets (as a percent)	16.70%