Investment Securities (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Held-to-maturity securities
Amortized cost	$ 2,400	$ 2,400
Estimated fair value	2,400	2,400
Carrying value	2,400	2,400
Available-for-sale securities
Amortized cost	5,213,646	5,423,189
Gross unrealized gains	79,540	119,670
Gross unrealized losses	(103,209)	(17,844)
Estimated fair value	5,189,977	5,525,015
Carrying value	5,189,977	5,525,015
Other securities
Held-to-maturity securities
Amortized cost	2,400	2,400
Estimated fair value	2,400	2,400
Carrying value	2,400	2,400
Residential mortgage-backed securities
Available-for-sale securities
Amortized cost	4,951,842	5,186,652
Gross unrealized gains	67,140	94,585
Gross unrealized losses	(98,004)	(16,033)
Estimated fair value	4,920,978	5,265,204
Carrying value	4,920,978	5,265,204
Mortgage-backed securities by Ginnie Mae
Available-for-sale securities
Carrying value	1,769,076	2,035,742
Mortgage-backed securities by Fannie Mae and Freddie Mac
Available-for-sale securities
Carrying value	3,121,461	3,196,602
Mortgage-backed securities by non-government entities
Available-for-sale securities
Carrying value	30,441	32,860
Obligations of states and political subdivisions
Available-for-sale securities
Amortized cost	242,229	216,962
Gross unrealized gains	11,372	23,504
Gross unrealized losses	(4,975)	(1,791)
Estimated fair value	248,626	238,675
Carrying value	248,626	238,675
Equity Securities
Available-for-sale securities
Amortized cost	19,575	19,575
Gross unrealized gains	1,028	1,581
Gross unrealized losses	(230)	(20)
Estimated fair value	20,373	21,136
Carrying value	$ 20,373	$ 21,136

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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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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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