Investment Securities (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 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	5,082,095	5,041,847
Gross unrealized gains	143,760	79,241
Gross unrealized Losses	(11,940)	(34,631)
Estimated fair value	5,213,915	5,086,457
Carrying value	5,213,915	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		1,327
Estimated fair value		1,327
Carrying value		1,327
Residential mortgage-backed securities
Available-for-sale securities
Amortized cost	4,851,747	4,876,573
Gross unrealized gains	128,196	77,741
Gross unrealized Losses	(10,680)	(29,846)
Estimated fair value	4,969,263	4,924,468
Carrying value	4,969,263	4,924,468
Mortgage-backed securities by Ginnie Mae
Available-for-sale securities
Carrying value	3,008,935	2,326,378
Mortgage-backed securities by Fannie Mae and Freddie Mac
Available-for-sale securities
Carrying value	1,920,723	2,552,062
Mortgage-backed securities by non-government entities
Available-for-sale securities
Carrying value	39,605	46,028
Obligations of states and political subdivisions
Available-for-sale securities
Amortized cost	211,523	150,122
Gross unrealized gains	14,449	636
Gross unrealized Losses	(1,211)	(4,761)
Estimated fair value	224,761	145,997
Carrying value	224,761	145,997
Equity Securities
Available-for-sale securities
Amortized cost	18,825	13,825
Gross unrealized gains	1,115	864
Gross unrealized Losses	(49)	(24)
Estimated fair value	19,891	14,665
Carrying value	$ 19,891	$ 14,665

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This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. This item represents Available-for-sale Securities which consist of all investments in certain debt and equity securities neither classified as trading or held-to-maturity securities. A debt security represents a creditor relationship with an enterprise. Debt securities include, among other items, US Treasury securities, US government securities, municipal securities, corporate bonds, convertible debt, commercial paper, and all securitized debt instruments. An equity security represents an ownership interest in an enterprise or the right to acquire or dispose of an ownership interest in an enterprise at fixed or determinable prices. Equity securities include, among other things, common stock, certain preferred stock, warrant rights, call options, and put options, but do not include convertible debt. An entity may opt to provide the reader with additional narrative text to better understand the nature of investments in debt and equity securities which are categorized as Available-for-sale.

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