Repurchase Agreements (Details) (USD $) In Thousands, unless otherwise specified	6 Months Ended	12 Months Ended
	Jun. 30, 2012 Agreements	Dec. 31, 2011 Agreements
Assets Sold under Agreements to Repurchase [Line Items]
Number of Agreement Borrowings Outstanding	20	20
Available Repurchase Agreement Counterparties	26
Debt, Weighted Average Interest Rate	0.62%	0.61%
Balance	$ 3,111,924	$ 2,093,793
Fair value of Collateral Pledged	3,363,353	2,268,312
Weighted Average Maturity of Agreements	50 days	57 days
Concentration Risk, Percentage	11.00%
30 days or less [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Balance	1,076,065	180,387
31 to 60 days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Balance	1,168,159	880,491
61 to 90 days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Balance	536,687	496,509
Greater than 90 days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Balance	331,013	536,406
Agency RMBS [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Debt, Weighted Average Interest Rate	0.40%	0.38%
Balance	2,162,480	1,447,508
Fair value of Collateral Pledged	2,239,357	1,521,107
Agency CMBS and CMBS IOs [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Debt, Weighted Average Interest Rate	0.81%	0.59%
Balance	468,322	290,362
Fair value of Collateral Pledged	536,268	329,612
Non-Agency RMBS [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Debt, Weighted Average Interest Rate	1.82%	1.85%
Balance	14,575	12,195
Fair value of Collateral Pledged	16,582	13,597
Non-Agency CMBS and CMBS IOs [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Debt, Weighted Average Interest Rate	1.41%	1.54%
Balance	425,241	283,266
Fair value of Collateral Pledged	520,751	336,124
Bonds [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Debt, Weighted Average Interest Rate	1.61%	1.65%
Balance	41,306	60,462
Fair value of Collateral Pledged	$ 50,395	$ 67,872