Mortgage-Backed Securities (Schedule Of Information About Mortgage Backed Securities That Will Reprice Or Amortize Based On Contractual Terms And Schedule Of Agency MBS Forward Purchase Commitments) (Details) (USD $) In Thousands, unless otherwise specified	9 Months Ended		12 Months Ended
	Sep. 30, 2011		Dec. 31, 2010
Mortgage Loans on Real Estate [Line Items]
Days To Coupon Reset Or Contractual Payment Minimum Range	30
Days To Coupon Reset Or Contractual Payment Maximum Range	90
Fair Value	$ 17,614,374		$ 9,587,216
% of Total	100.00%		100.00%
Weighted Average Coupon	3.54%		3.94%
Forward Purchase agreements - Face	1,214,182
Forward Purchase agreements - Cost	1,252,220
Forward Purchase agreements - Fair Market Value	1,253,194
Forward Purchase agreements - Due To Brokers	1,252,220	[1]
Forward Purchase agreements - Net Liability	974
Minimum [Member] | 0-18 Months [Member]
Mortgage Loans on Real Estate [Line Items]
Months to Coupon Reset or Contractual Payment	0		0
Minimum [Member] | 19-36 Months [Member]
Mortgage Loans on Real Estate [Line Items]
Months to Coupon Reset or Contractual Payment	19		19
Minimum [Member] | 37-60 Months [Member]
Mortgage Loans on Real Estate [Line Items]
Months to Coupon Reset or Contractual Payment	37		37
Minimum [Member] | 61-84 Months [Member]
Mortgage Loans on Real Estate [Line Items]
Months to Coupon Reset or Contractual Payment	61		61
Minimum [Member] | 85-120 Months [Member]
Mortgage Loans on Real Estate [Line Items]
Months to Coupon Reset or Contractual Payment	85		85
Minimum [Member] | 121 - 180 Months [Member]
Mortgage Loans on Real Estate [Line Items]
Months to Coupon Reset or Contractual Payment	121		121
Maximum [Member] | 0-18 Months [Member]
Mortgage Loans on Real Estate [Line Items]
Months to Coupon Reset or Contractual Payment	18		18
Maximum [Member] | 19-36 Months [Member]
Mortgage Loans on Real Estate [Line Items]
Months to Coupon Reset or Contractual Payment	36		36
Maximum [Member] | 37-60 Months [Member]
Mortgage Loans on Real Estate [Line Items]
Months to Coupon Reset or Contractual Payment	60		60
Maximum [Member] | 61-84 Months [Member]
Mortgage Loans on Real Estate [Line Items]
Months to Coupon Reset or Contractual Payment	84		84
Maximum [Member] | 85-120 Months [Member]
Mortgage Loans on Real Estate [Line Items]
Months to Coupon Reset or Contractual Payment	120		120
Maximum [Member] | 121 - 180 Months [Member]
Mortgage Loans on Real Estate [Line Items]
Months to Coupon Reset or Contractual Payment	180		180
0-18 Months [Member]
Mortgage Loans on Real Estate [Line Items]
Fair Value	1,158,144		632,979
% of Total	6.60%		6.60%
Weighted Average Coupon	4.76%		3.80%
19-36 Months [Member]
Mortgage Loans on Real Estate [Line Items]
Fair Value	962,674		1,628,967
% of Total	5.50%		17.00%
Weighted Average Coupon	4.47%		5.15%
37-60 Months [Member]
Mortgage Loans on Real Estate [Line Items]
Fair Value	8,666,670		5,195,155
% of Total	49.20%		54.20%
Weighted Average Coupon	3.41%		3.78%
61-84 Months [Member]
Mortgage Loans on Real Estate [Line Items]
Fair Value	5,389,188		1,938,935
% of Total	30.60%		20.20%
Weighted Average Coupon	3.40%		3.46%
85-120 Months [Member]
Mortgage Loans on Real Estate [Line Items]
Fair Value	307,070		191,180
% of Total	1.70%		2.00%
Weighted Average Coupon	3.62%		3.56%
121 - 180 Months [Member]
Mortgage Loans on Real Estate [Line Items]
Fair Value	$ 1,130,628
% of Total	6.40%
Weighted Average Coupon	3.13%

[1] Amounts due to brokers are usually settled within 30-90 days after period end.