Securities (Details) (USD $) In Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2011	Dec. 31, 2010
Securities available for sale:
Estimated Fair Market Value	$ 10,484,264
Mortgage-backed securities [Member]
Securities available for sale:
Amortized Cost	10,390,142	17,925,284
Gross Unrealized Gains	102,402	292,406
Gross Unrealized Losses	(8,280)	(97,513)
Estimated Fair Market Value	10,484,264	18,120,537
Mortgage-backed securities [Member] | GNMA pass-through certificates [Member]
Securities available for sale:
Amortized Cost	1,138,860	1,560,755
Gross Unrealized Gains	15,874	27,214
Gross Unrealized Losses	(1,114)	(7,487)
Estimated Fair Market Value	1,153,620	1,580,482
Mortgage-backed securities [Member] | FNMA pass-through certificates [Member]
Securities available for sale:
Amortized Cost	5,152,251	10,333,033
Gross Unrealized Gains	37,098	122,305
Gross Unrealized Losses	(5,270)	(57,550)
Estimated Fair Market Value	5,184,079	10,397,788
Mortgage-backed securities [Member] | FHLMC pass-through certificates [Member]
Securities available for sale:
Amortized Cost	4,006,482	5,521,741
Gross Unrealized Gains	48,234	129,547
Gross Unrealized Losses	(1,896)	(32,116)
Estimated Fair Market Value	4,052,820	5,619,172
Mortgage-backed securities [Member] | FHLMC and FNMA - REMICs [Member]
Securities available for sale:
Amortized Cost	92,549	509,755
Gross Unrealized Gains	1,196	13,340
Estimated Fair Market Value	93,745	523,095
Investment Securities [Member]
Securities available for sale:
Amortized Cost	6,767	86,767
Gross Unrealized Gains	454	3,028
Estimated Fair Market Value	7,221	89,795
Investment Securities [Member] | United States government-sponsored enterprise debt [Member]
Securities available for sale:
Amortized Cost		80,000
Gross Unrealized Gains		2,647
Estimated Fair Market Value		82,647
Investment Securities [Member] | Equity Securities [Member]
Securities available for sale:
Amortized Cost	6,767	6,767
Gross Unrealized Gains	454	381
Estimated Fair Market Value	$ 7,221	$ 7,148