Mortgage-Backed Securities (Schedule of Mortgage Backed Securities Portfolio Backed by U.S. Government Agency or U.S. Government Sponsored Entity) (Details) (USD $) In Thousands	Jun. 30, 2011	Dec. 31, 2010
MBS Amortized Cost	$ 16,157,729	$ 9,424,904
Gross unrealized losses	(10,297)	(33,125)
Gross unrealized gains	269,465	195,437
Fair Value	16,416,897	9,587,216
Fannie Mae Certificates [Member]
MBS Amortized Cost	11,466,783	6,537,472
Gross unrealized losses	(5,526)	(18,342)
Gross unrealized gains	200,270	138,626
Fair Value	11,661,527	6,657,756
Freddie Mac Certificates [Member]
MBS Amortized Cost	4,690,946	2,887,432
Gross unrealized losses	(4,771)	(14,783)
Gross unrealized gains	69,195	56,811
Fair Value	$ 4,755,370	$ 2,929,460