Investments In Securities And Interest Rate Swap And Cap Contracts (Agency RMBS Portfolio) (Details) (USD $)	12 Months Ended
	Dec. 31, 2011		Dec. 31, 2010
Financial Instruments Owned and Pledged as Collateral [Line Items]
Fair Value Disclosure Assets	9,472,094,000		6,371,145,000
Percentage of hybrid securities in portfolio	100.00%
10 Year Fixed Rate [Member]
Financial Instruments Owned and Pledged as Collateral [Line Items]
Number of year securities	10
15 Year Fixed Rate [Member]
Financial Instruments Owned and Pledged as Collateral [Line Items]
Number of year securities	15		15
20 Year Fixed Rate [Member]
Financial Instruments Owned and Pledged as Collateral [Line Items]
Number of year securities	20		20
30 Year Fixed Rate [Member]
Financial Instruments Owned and Pledged as Collateral [Line Items]
Number of year securities	30		30
Agency RMBS [Member]
Financial Instruments Owned and Pledged as Collateral [Line Items]
Par Value Disclosure Assets	8,925,617,000		6,156,908,000
Fair Value Disclosure Assets	9,372,454,000		6,310,570,000
Weighted Average, MTR	64.0	[1],[2]	63.2	[1],[2]
Weighted Average, Coupon	3.66%		3.83%
Weighted Average, CPR	19.50%	[3]	18.90%	[3]
Agency RMBS [Member] | Par Value [Member]
Financial Instruments Owned and Pledged as Collateral [Line Items]
Weighted Average, Cost/Par	102.50		102.46
Agency RMBS [Member] | 10 Year Fixed Rate [Member]
Financial Instruments Owned and Pledged as Collateral [Line Items]
Par Value Disclosure Assets	272,115,000
Fair Value Disclosure Assets	284,948,000
Weighted Average, Cost/Par	103.96
Weighted Average, Fair Value/Par	104.72
Weighted Average, Coupon	3.50%
Weighted Average, CPR	13.60%	[3]
Agency RMBS [Member] | 15 Year Fixed Rate [Member]
Financial Instruments Owned and Pledged as Collateral [Line Items]
Par Value Disclosure Assets	4,763,965,000		3,549,194,000
Fair Value Disclosure Assets	5,010,121,000		3,622,862,000
Weighted Average, Cost/Par	102.53		102.16
Weighted Average, Fair Value/Par	105.17		102.08
Weighted Average, Coupon	3.79%		3.87%
Weighted Average, CPR	17.80%	[3]	23.10%	[3]
Agency RMBS [Member] | 20 Year Fixed Rate [Member]
Financial Instruments Owned and Pledged as Collateral [Line Items]
Par Value Disclosure Assets	551,766,000		647,360,000
Fair Value Disclosure Assets	585,103,000		660,237,000
Weighted Average, Cost/Par	102.32		102.38
Weighted Average, Fair Value/Par	106.04		101.99
Weighted Average, Coupon	4.14%		4.14%
Weighted Average, CPR	28.10%	[3]	6.90%	[3]
Agency RMBS [Member] | 30 Year Fixed Rate [Member]
Financial Instruments Owned and Pledged as Collateral [Line Items]
Par Value Disclosure Assets	239,747,000		223,047,000
Fair Value Disclosure Assets	259,123,000		238,549,000
Weighted Average, Cost/Par	103.09		105.60
Weighted Average, Fair Value/Par	108.08		106.95
Weighted Average, Coupon	5.00%		5.55%
Weighted Average, CPR	26.30%	[3]	28.20%	[3]
Agency RMBS [Member] | Hybrid ARMs [Member]
Financial Instruments Owned and Pledged as Collateral [Line Items]
Par Value Disclosure Assets	3,098,024,000		1,737,307,000
Fair Value Disclosure Assets	3,233,159,000		1,788,922,000
Weighted Average, Cost/Par	102.31		102.70
Weighted Average, Fair Value/Par	104.36		102.97
Weighted Average, MTR	64.0	[1]	63.2	[1]
Weighted Average, Coupon	3.29%		3.43%
Weighted Average, CPR	20.30%	[3]	18.40%	[3]
Agency RMBS [Member] | Fair Value [Member]
Financial Instruments Owned and Pledged as Collateral [Line Items]
Weighted Average, Fair Value/Par	105.01		102.50

[1] MTR, or "Months to Reset" is the number of months remaining before the fixed rate on a hybrid ARM becomes a variable rate. At the end of the fixed period, the variable rate will be determined by the margin and the pre-specified caps of the ARM. After the fixed period, 100% of the hybrid ARMS in the portfolio reset annually.

[2] Weighted average months to reset of our Hybrid ARM portfolio.

[3] CPR, or "Constant Prepayment Rate," is a method of expressing the prepayment rate for a mortgage pool that assumes that a constant fraction of the remaining principal is prepaid each month or year. Specifically, the constant prepayment rate is an annualized version of the prior three month prepayment rate. Securities with no prepayment history are excluded from this calculation.