Investment Securities (Details) (USD $) In Thousands	Jun. 30, 2011	Dec. 31, 2010
Held-to-maturity securities
Amortized cost	$ 2,450	$ 2,450
Estimated fair value	2,450	2,450
Carrying value	2,450	2,450
Available-for-sale securities
Amortized cost	4,933,681	5,041,847
Gross unrealized gains	146,281	79,241
Gross unrealized Losses	(12,314)	(34,631)
Estimated fair value	5,067,648	5,086,457
Carrying value	5,067,648	5,086,457
Other securities
Held-to-maturity securities
Amortized cost	2,450	2,450
Estimated fair value	2,450	2,450
Carrying value	2,450	2,450
U.S. Treasury securities
Available-for-sale securities
Amortized cost	828	1,327
Estimated fair value	828	1,327
Carrying value	828	1,327
Residential mortgage-backed securities
Available-for-sale securities
Amortized cost	4,716,507	4,876,573
Gross unrealized gains	138,394	77,741
Gross unrealized Losses	(11,300)	(29,846)
Estimated fair value	4,843,601	4,924,468
Carrying value	4,843,601	4,924,468
Mortgage-backed securities by Ginnie Mae
Available-for-sale securities
Carrying value	3,031,788	2,326,378
Mortgage-backed securities by Fannie Mae and Freddie Mac
Available-for-sale securities
Carrying value	1,770,125	2,552,062
Mortgage-backed securities by non-government entities
Available-for-sale securities
Carrying value	41,688	46,028
Obligations of states and political subdivisions
Available-for-sale securities
Amortized cost	202,521	150,122
Gross unrealized gains	6,938	636
Gross unrealized Losses	(1,001)	(4,761)
Estimated fair value	208,458	145,997
Carrying value	208,458	145,997
Equity Securities
Available-for-sale securities
Amortized cost	13,825	13,825
Gross unrealized gains	949	864
Gross unrealized Losses	(13)	(24)
Estimated fair value	14,761	14,665
Carrying value	$ 14,761	$ 14,665

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