Derivative Instruments (Details 3) (USD $)	3 Months Ended			9 Months Ended
	Sep. 30, 2013 derivative		Sep. 30, 2012 derivative	Sep. 30, 2013 derivative		Sep. 30, 2012 derivative	Dec. 31, 2012
Impact of Hedging Instruments on AOCI
Balance at beginning of period	$ (31,967,000)		$ (89,823,000)	$ (62,831,000)		$ (114,194,000)
Unrealized (loss)/gain on Swaps, net	(19,934,000)		11,654,000	10,930,000		36,025,000
Balance at end of period	(51,901,000)		(78,169,000)	(51,901,000)		(78,169,000)
Assets Pledged	10,315,995,000			10,315,995,000			10,624,276,000
Restricted cash	59,275,000			59,275,000			5,016,000
Number of derivatives terminated	0		0	0		0
Aggregate notional amount of derivatives	350,000,000			350,000,000			0
Interest expense attributable to Swaps	41,950,000		45,801,000	120,586,000		128,616,000
TBA short positions, cost basis	343,436,000	[1]		343,436,000	[1]		0	[1]
TBA short positions, at fair value	352,160,000	[2]		352,160,000	[2]		0	[2]
TBA short positions, at fair value	(8,724,000)	[3]		(8,724,000)	[3]		0	[3]
Interest Rate Contract [Member]
Impact of Hedging Instruments on AOCI
Assets Pledged	126,530,000			126,530,000			73,931,000
Swaps
Impact of Hedging Instruments on AOCI
Balance at end of period	51,900,000			51,900,000
Aggregate fair value of assets needed to immediately settle	59,200,000			59,200,000
Associated accrued interest payable	2,600,000			2,600,000
Aggregate notional amount of derivatives	4,079,263,000	[4]		4,079,263,000	[4]		2,519,564,000	[4]
Interest rate, description				one-month or three-month London Interbank Offered Rate (LIBOR)
Average maturity term of swaps				51 months
Maximum maturity term of swaps				9 years 11 months 9 days
Weighted Average Fixed-Pay Interest Rate (as a percent)	1.93%	[4]		1.93%	[4]		2.31%	[4]
Weighted Average Variable Interest Rate (as a percent)	0.18%	[4],[5]		0.18%	[4],[5]		0.22%	[4],[5]
Interest expense attributable to Swaps	15,888,000		18,138,000	40,884,000		58,275,000
Weighted average Swap rate paid (as a percent)	2.07%		2.72%	2.15%		2.74%
Weighted average Swap rate received (as a percent)	0.20%		0.27%	0.20%		0.29%
Swaps | Minimum | LIBOR
Impact of Hedging Instruments on AOCI
Derivative, variable interest rate, term				1 month
Swaps | Maximum | LIBOR
Impact of Hedging Instruments on AOCI
Derivative, variable interest rate, term				3 months
Swaps | Within 30 days.
Impact of Hedging Instruments on AOCI
Aggregate notional amount of derivatives	14,509,000	[4]		14,509,000	[4]		25,828,000	[4]
Weighted Average Fixed-Pay Interest Rate (as a percent)	4.02%	[4]		4.02%	[4]		3.88%	[4]
Weighted Average Variable Interest Rate (as a percent)	0.23%	[4],[5]		0.23%	[4],[5]		0.28%	[4],[5]
Swaps | Over 30 days to 3 months
Impact of Hedging Instruments on AOCI
Aggregate notional amount of derivatives	19,542,000	[4]		19,542,000	[4]		30,185,000	[4]
Weighted Average Fixed-Pay Interest Rate (as a percent)	4.16%	[4]		4.16%	[4]		3.96%	[4]
Weighted Average Variable Interest Rate (as a percent)	0.21%	[4],[5]		0.21%	[4],[5]		0.26%	[4],[5]
Swaps | Over 3 months to 6 months
Impact of Hedging Instruments on AOCI
Aggregate notional amount of derivatives	41,851,000	[4]		41,851,000	[4]		527,275,000	[4]
Weighted Average Fixed-Pay Interest Rate (as a percent)	3.92%	[4]		3.92%	[4]		1.63%	[4]
Weighted Average Variable Interest Rate (as a percent)	0.23%	[4],[5]		0.23%	[4],[5]		0.21%	[4],[5]
Swaps | Over 6 months to 12 months
Impact of Hedging Instruments on AOCI
Aggregate notional amount of derivatives	522,469,000	[4]		522,469,000	[4]		391,063,000	[4]
Weighted Average Fixed-Pay Interest Rate (as a percent)	1.90%	[4]		1.90%	[4]		4.17%	[4]
Weighted Average Variable Interest Rate (as a percent)	0.18%	[4],[5]		0.18%	[4],[5]		0.22%	[4],[5]
Swaps | Over 12 months to 24 months
Impact of Hedging Instruments on AOCI
Aggregate notional amount of derivatives	830,892,000	[4]		830,892,000	[4]		685,042,000	[4]
Weighted Average Fixed-Pay Interest Rate (as a percent)	2.17%	[4]		2.17%	[4]		2.28%	[4]
Weighted Average Variable Interest Rate (as a percent)	0.19%	[4],[5]		0.19%	[4],[5]		0.22%	[4],[5]
Swaps | Over 24 months to 36 months
Impact of Hedging Instruments on AOCI
Aggregate notional amount of derivatives	50,000,000	[4]		50,000,000	[4]		710,171,000	[4]
Weighted Average Fixed-Pay Interest Rate (as a percent)	2.13%	[4]		2.13%	[4]		1.97%	[4]
Weighted Average Variable Interest Rate (as a percent)	0.18%	[4],[5]		0.18%	[4],[5]		0.21%	[4],[5]
Swaps | Over 36 months to 48 months
Impact of Hedging Instruments on AOCI
Aggregate notional amount of derivatives	450,000,000	[4]		450,000,000	[4]		150,000,000	[4]
Weighted Average Fixed-Pay Interest Rate (as a percent)	0.56%	[4]		0.56%	[4]		1.03%	[4]
Weighted Average Variable Interest Rate (as a percent)	0.18%	[4],[5]		0.18%	[4],[5]		0.21%	[4],[5]
Swaps | Over 48 Months to 60 Months
Impact of Hedging Instruments on AOCI
Aggregate notional amount of derivatives	550,000,000	[4]		550,000,000	[4]		0	[4]
Weighted Average Fixed-Pay Interest Rate (as a percent)	1.49%	[4]		1.49%	[4]		0.00%	[4]
Weighted Average Variable Interest Rate (as a percent)	0.18%	[4],[5]		0.18%	[4],[5]		0.00%	[4],[5]
Swaps | Over 60 Months to 72 Months
Impact of Hedging Instruments on AOCI
Aggregate notional amount of derivatives	0	[4]		0	[4]		0	[4]
Weighted Average Fixed-Pay Interest Rate (as a percent)	0.00%	[4]		0.00%	[4]		0.00%	[4]
Weighted Average Variable Interest Rate (as a percent)	0.00%	[4],[5]		0.00%	[4],[5]		0.00%	[4],[5]
Swaps | Over 72 Months to 84 Months
Impact of Hedging Instruments on AOCI
Aggregate notional amount of derivatives	1,500,000,000	[4]		1,500,000,000	[4]		0	[4]
Weighted Average Fixed-Pay Interest Rate (as a percent)	2.22%	[4]		2.22%	[4]		0.00%	[4]
Weighted Average Variable Interest Rate (as a percent)	0.18%	[4],[5]		0.18%	[4],[5]		0.00%	[4],[5]
Swaps | Over 84 Months
Impact of Hedging Instruments on AOCI
Aggregate notional amount of derivatives	100,000,000	[4],[6]		100,000,000	[4],[6]		0	[4],[6]
Weighted Average Fixed-Pay Interest Rate (as a percent)	2.75%	[4],[6]		2.75%	[4],[6]		0.00%	[4],[6]
Weighted Average Variable Interest Rate (as a percent)	0.18%	[4],[5],[6]		0.18%	[4],[5],[6]		0.00%	[4],[5],[6]
Agency MBS, at fair value | Interest Rate Contract [Member]
Impact of Hedging Instruments on AOCI
Assets Pledged	67,255,000			67,255,000			68,915,000
Cash and Cash Equivalents [Domain] | Interest Rate Contract [Member]
Impact of Hedging Instruments on AOCI
Restricted cash	59,275,000			59,275,000			5,016,000
New Swaps [Member] | Swaps | Minimum
Impact of Hedging Instruments on AOCI
Initial Term of Derivative Entered	5 years			2 months
New Swaps [Member] | Swaps | Maximum
Impact of Hedging Instruments on AOCI
Initial Term of Derivative Entered	10 years			10 years
New Swaps [Member] | Swaps | QTD
Impact of Hedging Instruments on AOCI
Number of new swaps	14
Notional amount of new swaps	1,750,000,000
Weighted Average Fixed-Pay Interest Rate (as a percent)	2.13%			2.13%
New Swaps [Member] | Swaps | YTD
Impact of Hedging Instruments on AOCI
Number of new swaps				23
Notional amount of new swaps				2,501,000,000
Weighted Average Fixed-Pay Interest Rate (as a percent)	1.85%			1.85%
Swaps amortized expired [Member] | Swaps | QTD
Impact of Hedging Instruments on AOCI
Weighted Average Fixed-Pay Interest Rate (as a percent)	4.16%			4.16%
Aggregate notional amount of derivatives expired	358,012,000
Swaps amortized expired [Member] | Swaps | YTD
Impact of Hedging Instruments on AOCI
Weighted Average Fixed-Pay Interest Rate (as a percent)	2.73%			2.73%
Aggregate notional amount of derivatives expired				$ 941,300,000

[1] Cost basis represents the forward price to be received for the underlying agency security.

[2] Fair value represents the current fair value of the underlying agency security as of period-end.

[3] Net carrying value represents the difference between the fair value of the underlying agency security as of period-end and the cost basis and is reported in Other derivative instruments, at fair value in the Company's consolidated balance sheets. Gains and losses associated with TBA short positions are reported in Other income on the Company's consolidated statement of operations.

[4] Each maturity category reflects contractual amortization and/or maturity of notional amounts.

[5] Reflects the benchmark variable rate due from the counterparty at the date presented, which rate adjusts monthly or quarterly based on one-month or three-month LIBOR, respectively.

[6] Reflects one Swap with a maturity date of July 2023.