Agency MBS (Details) (Agency MBS [Member], USD $)	3 Months Ended			9 Months Ended
	Sep. 30, 2012		Sep. 30, 2011	Sep. 30, 2012		Sep. 30, 2011	Dec. 31, 2011
Principal/par value	$ 2,917,026,000			$ 2,917,026,000			$ 1,755,349,000
Unamortized premium	676,146,000			676,146,000			193,473,000
Unamortized discount	(14,000)			(14,000)			(17,000)
Amortized cost	3,593,158,000			3,593,158,000			1,948,805,000
Available for sale securities, gross unrealized gains	64,038,000			64,038,000			22,883,000
Available for sale securities, gross unrealized losses	(9,123,000)			(9,123,000)			(8,448,000)
Trading securities gross unrealized gains	2,599,000			2,599,000			1,919,000
Total MBS investments at fair value	3,650,672,000			3,650,672,000			1,965,159,000
RMBS [Member]
Principal/par value	2,627,300,000			2,627,300,000			1,488,397,000
Unamortized premium	145,922,000			145,922,000			85,488,000
Unamortized discount	(14,000)			(14,000)			(17,000)
Amortized cost	2,773,208,000			2,773,208,000			1,573,868,000
Available for sale securities, gross unrealized gains	32,315,000			32,315,000			10,787,000
Available for sale securities, gross unrealized losses	(5,428,000)			(5,428,000)			(7,405,000)
Trading securities gross unrealized gains	0			0			0
Total MBS investments at fair value	2,800,095,000			2,800,095,000			1,577,250,000
Weighted average coupon, MBS	3.73%			3.73%			4.54%
Payments to Acquire Mortgage Backed Securities (MBS)				1,707,077,000
CMBS [Member]
Principal/par value	289,726,000			289,726,000			266,952,000
Unamortized premium	21,898,000			21,898,000			21,627,000
Unamortized discount	0			0			0
Amortized cost	311,624,000			311,624,000			288,579,000
Available for sale securities, gross unrealized gains	20,843,000			20,843,000			11,746,000
Available for sale securities, gross unrealized losses	(2,000)			(2,000)			0
Trading securities gross unrealized gains	2,599,000			2,599,000			1,919,000
Total MBS investments at fair value	335,064,000			335,064,000			302,244,000
Weighted average coupon, MBS	5.20%			5.20%			5.20%
Payments to Acquire Mortgage Backed Securities (MBS)				477,774,000
Trading Securities, Cost	27,687,000			27,687,000			28,119,000
Trading Securities, Change in Unrealized Holding Gain (Loss)	283,000		744,000	716,000		1,819,000
CMBS - IO [Member]
Principal/par value	0	[1]		0	[1]		0	[2]
Unamortized premium	508,326,000	[1]		508,326,000	[1]		86,358,000	[2]
Unamortized discount	0	[1]		0	[1]		0	[2]
Amortized cost	508,326,000	[1]		508,326,000	[1]		86,358,000	[2]
Available for sale securities, gross unrealized gains	10,880,000	[1]		10,880,000	[1]		350,000	[2]
Available for sale securities, gross unrealized losses	(3,693,000)	[1]		(3,693,000)	[1]		(1,043,000)	[2]
Trading securities gross unrealized gains	0	[1]		0	[1]		0	[2]
Total MBS investments at fair value	515,513,000	[1]		515,513,000	[1]		85,665,000	[2]
Weighted average coupon, MBS	0.96%	[1]		0.96%	[1]		0.96%	[2]
Notional balance of Agency IO investments	$ 9,251,338,000			$ 9,251,338,000			$ 1,813,096,000

[1] The combined notional balance for the Agency CMBS IO securities is $9,251,338 as of September 30, 2012.

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