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	6 Months Ended		12 Months Ended
	Jun. 30, 2011		Dec. 31, 2010
Days To Coupon Reset Or Contractual Payment Minimum Range	30
Days To Coupon Reset Or Contractual Payment Maximum Range	90
Fair Value	$ 16,416,897		$ 9,587,216
% of Total	100.00%		100.00%
Weighted Average Coupon	3.61%		3.94%
Forward purchase agreements - Face	230,000
Forward purchase agreements - Cost	236,457
Forward purchase agreements - Fair Market Value	236,392
Forward purchase agreements - Due to Brokers	236,457	[1]
Forward purchase agreements - Net Liability	(65)
Minimum [Member] | 0-18 Months [Member]
Months to Coupon Reset or Contractual Payment	0		0
Minimum [Member] | 19-36 Months [Member]
Months to Coupon Reset or Contractual Payment	19		19
Minimum [Member] | 37-60 Months [Member]
Months to Coupon Reset or Contractual Payment	37		37
Minimum [Member] | 61-84 Months [Member]
Months to Coupon Reset or Contractual Payment	61		61
Minimum [Member] | 85-120 Months [Member]
Months to Coupon Reset or Contractual Payment	85		85
Maximum [Member] | 0-18 Months [Member]
Months to Coupon Reset or Contractual Payment	18		18
Maximum [Member] | 19-36 Months [Member]
Months to Coupon Reset or Contractual Payment	36		36
Maximum [Member] | 37-60 Months [Member]
Months to Coupon Reset or Contractual Payment	60		60
Maximum [Member] | 61-84 Months [Member]
Months to Coupon Reset or Contractual Payment	84		84
Maximum [Member] | 85-120 Months [Member]
Months to Coupon Reset or Contractual Payment	120		120
0-18 Months [Member]
Fair Value	1,003,150		632,979
% of Total	6.00%		6.60%
Weighted Average Coupon	4.31%		3.80%
19-36 Months [Member]
Fair Value	1,157,422		1,628,967
% of Total	7.10%		17.00%
Weighted Average Coupon	4.75%		5.15%
37-60 Months [Member]
Fair Value	9,136,269		5,195,156
% of Total	55.70%		54.20%
Weighted Average Coupon	3.47%		3.78%
61-84 Months [Member]
Fair Value	4,944,690		1,938,935
% of Total	30.10%		20.20%
Weighted Average Coupon	3.45%		3.46%
85-120 Months [Member]
Fair Value	$ 175,366		$ 191,180
% of Total	1.10%		2.00%
Weighted Average Coupon	3.98%		3.56%

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