Investment Securities (Details) (USD $) In Thousands	Sep. 30, 2011	Dec. 31, 2010
Held-to-maturity securities
Amortized cost	$ 2,575	$ 2,450
Estimated fair value	2,575	2,450
Carrying value	2,575	2,450
Available-for-sale securities
Amortized cost	4,840,723	5,041,847
Gross unrealized gains	160,308	79,241
Gross unrealized Losses	(12,118)	(34,631)
Estimated fair value	4,988,913	5,086,457
Carrying value	4,988,913	5,086,457
Other securities
Held-to-maturity securities
Amortized cost	2,575	2,450
Estimated fair value	2,575	2,450
Carrying value	2,575	2,450
U.S. Treasury securities
Available-for-sale securities
Amortized cost		1,327
Estimated fair value		1,327
Carrying value		1,327
Residential mortgage-backed securities
Available-for-sale securities
Amortized cost	4,616,720	4,876,573
Gross unrealized gains	144,020	77,741
Gross unrealized Losses	(11,159)	(29,846)
Estimated fair value	4,749,581	4,924,468
Carrying value	4,749,581	4,924,468
Mortgage-backed securities by Ginnie Mae
Available-for-sale securities
Carrying value	2,958,000	2,326,378
Mortgage-backed securities by Fannie Mae and Freddie Mac
Available-for-sale securities
Carrying value	1,751,104	2,552,062
Mortgage-backed securities by non-government entities
Available-for-sale securities
Carrying value	40,477	46,028
Obligations of states and political subdivisions
Available-for-sale securities
Amortized cost	210,178	150,122
Gross unrealized gains	15,257	636
Gross unrealized Losses	(920)	(4,761)
Estimated fair value	224,515	145,997
Carrying value	224,515	145,997
Equity Securities
Available-for-sale securities
Amortized cost	13,825	13,825
Gross unrealized gains	1,031	864
Gross unrealized Losses	(39)	(24)
Estimated fair value	14,817	14,665
Carrying value	$ 14,817	$ 14,665

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