Investment Securities (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Held-to-maturity securities
Amortized Cost	$ 2,425	$ 2,450
Estimated Fair Value	2,425	2,450
Carrying Value	2,425	2,450
Available-for-sale securities
Amortized Cost	5,077,867	5,082,095
Gross Unrealized Gains	146,390	143,760
Gross Unrealized Losses	(15,654)	(11,940)
Estimated Fair Value	5,208,603	5,213,915
Carrying Value	5,208,603	5,213,915
Other securities
Held-to-maturity securities
Amortized Cost	2,425	2,450
Estimated Fair Value	2,425	2,450
Carrying Value	2,425	2,450
Residential mortgage-backed securities
Available-for-sale securities
Amortized Cost	4,841,592	4,851,747
Gross Unrealized Gains	126,267	128,196
Gross Unrealized Losses	(14,265)	(10,680)
Estimated Fair Value	4,953,594	4,969,263
Carrying Value	4,953,594	4,969,263
Mortgage-backed securities by Ginnie Mae
Available-for-sale securities
Carrying Value	2,654,113	3,008,935
Mortgage-backed securities by Fannie Mae and Freddie Mac
Available-for-sale securities
Carrying Value	2,263,605	1,920,723
Mortgage-backed securities by non-government entities
Available-for-sale securities
Carrying Value	35,876	39,605
Obligations of states and political subdivisions
Available-for-sale securities
Amortized Cost	216,700	211,523
Gross Unrealized Gains	18,563	14,449
Gross Unrealized Losses	(1,364)	(1,211)
Estimated Fair Value	233,899	224,761
Carrying Value	233,899	224,761
Equity Securities
Available-for-sale securities
Amortized Cost	19,575	18,825
Gross Unrealized Gains	1,560	1,115
Gross Unrealized Losses	(25)	(49)
Estimated Fair Value	21,110	19,891
Carrying Value	$ 21,110	$ 19,891

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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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