Mortgage-Backed Securities (Schedule Of Information About Mortgage-Backed Securities That Will Reprice Or Amortize Based On Contractual Terms) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010
Mortgage Loans on Real Estate [Line Items]
Fair Value	$ 17,741,873	$ 9,587,216
% of Total	100.00%	100.00%
Weighted Average Coupon	3.46%	3.94%
0- 18 Months [Member]
Mortgage Loans on Real Estate [Line Items]
Fair Value	1,443,135	632,979
% of Total	8.10%	6.60%
Weighted Average Coupon	4.75%	3.80%
19- 36 Months [Member]
Mortgage Loans on Real Estate [Line Items]
Fair Value	1,010,209	1,628,967
% of Total	5.70%	17.00%
Weighted Average Coupon	3.95%	5.15%
37- 60 Months [Member]
Mortgage Loans on Real Estate [Line Items]
Fair Value	8,108,775	5,195,155
% of Total	45.70%	54.20%
Weighted Average Coupon	3.33%	3.78%
61- 84 Months [Member]
Mortgage Loans on Real Estate [Line Items]
Fair Value	6,036,655	1,938,935
% of Total	34.00%	20.20%
Weighted Average Coupon	3.32%	3.46%
85- 120 Months [Member]
Mortgage Loans on Real Estate [Line Items]
Fair Value	155,160	191,180
% of Total	0.90%	2.00%
Weighted Average Coupon	3.96%	3.56%
121- 180 Months [Member]
Mortgage Loans on Real Estate [Line Items]
Fair Value	$ 987,939
% of Total	5.60%
Weighted Average Coupon	3.05%
Minimum [Member] | 0- 18 Months [Member]
Mortgage Loans on Real Estate [Line Items]
Months to Coupon Reset or Contractual Payment	0
Minimum [Member] | 19- 36 Months [Member]
Mortgage Loans on Real Estate [Line Items]
Months to Coupon Reset or Contractual Payment	19
Minimum [Member] | 37- 60 Months [Member]
Mortgage Loans on Real Estate [Line Items]
Months to Coupon Reset or Contractual Payment	37
Minimum [Member] | 61- 84 Months [Member]
Mortgage Loans on Real Estate [Line Items]
Months to Coupon Reset or Contractual Payment	61
Minimum [Member] | 85- 120 Months [Member]
Mortgage Loans on Real Estate [Line Items]
Months to Coupon Reset or Contractual Payment	85
Minimum [Member] | 121- 180 Months [Member]
Mortgage Loans on Real Estate [Line Items]
Months to Coupon Reset or Contractual Payment	121
Maximum [Member] | 0- 18 Months [Member]
Mortgage Loans on Real Estate [Line Items]
Months to Coupon Reset or Contractual Payment	18
Maximum [Member] | 19- 36 Months [Member]
Mortgage Loans on Real Estate [Line Items]
Months to Coupon Reset or Contractual Payment	36
Maximum [Member] | 37- 60 Months [Member]
Mortgage Loans on Real Estate [Line Items]
Months to Coupon Reset or Contractual Payment	60
Maximum [Member] | 61- 84 Months [Member]
Mortgage Loans on Real Estate [Line Items]
Months to Coupon Reset or Contractual Payment	84
Maximum [Member] | 85- 120 Months [Member]
Mortgage Loans on Real Estate [Line Items]
Months to Coupon Reset or Contractual Payment	120
Maximum [Member] | 121- 180 Months [Member]
Mortgage Loans on Real Estate [Line Items]
Months to Coupon Reset or Contractual Payment	180