Investment Securities (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011
Investment securities available for sale:
Total investment securities available for sale, Amortized Cost	$ 4,508,971	$ 4,775,400
Total investment securities available for sale, Gross unrealized gains	161,960	165,746
Total investment securities available for sale, Gross unrealized losses	(1,831)	(3,663)
Investment securities available for sale, at fair value	4,669,100	4,937,483
US Treasury Securities [Member]
Investment securities available for sale:
Total investment securities available for sale, Amortized Cost	1,005	1,000
Total investment securities available for sale, Gross unrealized gains	0	1
Total investment securities available for sale, Gross unrealized losses	(4)	0
Investment securities available for sale, at fair value	1,001	1,001
Federal agency securities [Member]
Investment securities available for sale:
Total investment securities available for sale, Amortized Cost	7	24,031
Total investment securities available for sale, Gross unrealized gains	1	18
Total investment securities available for sale, Gross unrealized losses	0	0
Investment securities available for sale, at fair value	8	24,049
Obligations of state and political subdivisions (municipal securities) [Member]
Investment securities available for sale:
Total investment securities available for sale, Amortized Cost	792,854	797,691
Total investment securities available for sale, Gross unrealized gains	43,068	49,583
Total investment securities available for sale, Gross unrealized losses	(258)	(28)
Investment securities available for sale, at fair value	835,664	847,246
Residential mortgage-related securities [Member]
Investment securities available for sale:
Total investment securities available for sale, Amortized Cost	3,440,370	3,674,696
Total investment securities available for sale, Gross unrealized gains	114,939	112,357
Total investment securities available for sale, Gross unrealized losses	(879)	(1,463)
Investment securities available for sale, at fair value	3,554,430	3,785,590
Commercial mortgage-related securities [Member]
Investment securities available for sale:
Total investment securities available for sale, Amortized Cost	17,779	16,647
Total investment securities available for sale, Gross unrealized gains	1,524	1,896
Total investment securities available for sale, Gross unrealized losses	0	0
Investment securities available for sale, at fair value	19,303	18,543
Asset-backed securities [Member]
Investment securities available for sale:
Total investment securities available for sale, Amortized Cost	163,466	188,439
Total investment securities available for sale, Gross unrealized gains	1	0
Total investment securities available for sale, Gross unrealized losses	(453)	(707)
Investment securities available for sale, at fair value	163,014	187,732
Other securities (debt and equity) [Member]
Investment securities available for sale:
Total investment securities available for sale, Amortized Cost	93,490	72,896
Total investment securities available for sale, Gross unrealized gains	2,427	1,891
Total investment securities available for sale, Gross unrealized losses	(237)	(1,465)
Investment securities available for sale, at fair value	$ 95,680	$ 73,322

This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

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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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- Definition

This item represents the gross unrealized gains for securities, at a point in time, which are categorized neither as held-to-maturity nor trading securities.

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