Agency MBS (Details) (Agency MBS [Member], USD $)	3 Months Ended			6 Months Ended
	Jun. 30, 2012		Jun. 30, 2011	Jun. 30, 2012		Jun. 30, 2011	Dec. 31, 2011
Principal/par value	$ 2,503,339,000			$ 2,503,339,000			$ 1,755,349,000
Unamortized premium	438,286,000			438,286,000			193,473,000
Unamortized discount	(15,000)			(15,000)			(17,000)
Amortized cost	2,941,610,000			2,941,610,000			1,948,805,000
Available for sale securities, gross unrealized gains	42,348,000			42,348,000			22,883,000
Available for sale securities, gross unrealized losses	(7,235,000)			(7,235,000)			(8,448,000)
Trading securities gross unrealized gains	2,316,000			2,316,000			1,919,000
Total MBS investments at fair value	2,979,039,000			2,979,039,000			1,965,159,000
Weighted average coupon, MBS	4.02%			4.02%			4.64%
RMBS [Member]
Principal/par value	2,216,357,000			2,216,357,000			1,488,397,000
Unamortized premium	120,828,000			120,828,000			85,488,000
Unamortized discount	(15,000)			(15,000)			(17,000)
Amortized cost	2,337,170,000			2,337,170,000			1,573,868,000
Available for sale securities, gross unrealized gains	19,409,000			19,409,000			10,787,000
Available for sale securities, gross unrealized losses	(6,175,000)			(6,175,000)			(7,405,000)
Trading securities gross unrealized gains	0			0			0
Total MBS investments at fair value	2,350,404,000			2,350,404,000			1,577,250,000
Weighted average coupon, MBS	3.87%			3.87%			4.54%
Payments to Acquire Mortgage Backed Securities (MBS)				1,020,900,000
CMBS [Member]
Principal/par value	286,982,000			286,982,000			266,952,000
Unamortized premium	22,412,000			22,412,000			21,627,000
Unamortized discount	0			0			0
Amortized cost	309,394,000			309,394,000			288,579,000
Available for sale securities, gross unrealized gains	17,047,000			17,047,000			11,746,000
Available for sale securities, gross unrealized losses	0			0			0
Trading securities gross unrealized gains	2,316,000			2,316,000			1,919,000
Total MBS investments at fair value	328,757,000			328,757,000			302,244,000
Weighted average coupon, MBS	5.19%			5.19%			5.20%
Payments to Acquire Mortgage Backed Securities (MBS)				247,481,000
Trading Securities, Cost	27,833,000			27,833,000			28,119,000
Trading Securities, Change in Unrealized Holding Gain (Loss)	569,000		956,000	433,000		1,075,000
CMBS - IO [Member]
Principal/par value	0	[1]		0	[1]		0	[2]
Unamortized premium	295,046,000	[1]		295,046,000	[1]		86,358,000	[2]
Unamortized discount	0	[1]		0	[1]		0	[2]
Amortized cost	295,046,000	[1]		295,046,000	[1]		86,358,000	[2]
Available for sale securities, gross unrealized gains	5,892,000	[1]		5,892,000	[1]		350,000	[2]
Available for sale securities, gross unrealized losses	(1,060,000)	[1]		(1,060,000)	[1]		(1,043,000)	[2]
Trading securities gross unrealized gains	0	[1]		0	[1]		0	[2]
Total MBS investments at fair value	299,878,000	[1]		299,878,000	[1]		85,665,000	[2]
Weighted average coupon, MBS	1.17%	[1]		1.17%	[1]		0.96%	[2]
Notional balance of Agency IO investments	$ 4,543,349,000			$ 4,543,349,000			$ 1,813,096,000

[1] The combined notional balance for the Agency CMBS IO securities is $4,543,349 as of June 30, 2012.

[2] The combined notional balance for the Agency CMBS IO securities is $1,813,096 as of December 31, 2011,