Securities Available for Sale (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Securities available for sale, Amortized Cost	$ 6,739,736	$ 6,802,656
Securities available for sale, Gross Unrealized Gains	112,051	139,649
Securities available for sale, Gross Unrealized Losses	(10,203)	(4,842)
Total securities available for sale, Fair Value	6,841,584	6,937,463
U.S. Treasury [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities available for sale, Amortized Cost	121,777	116,856
Securities available for sale, Gross Unrealized Gains	1,260	1,166
Securities available for sale, Gross Unrealized Losses	(216)	(171)
Total securities available for sale, Fair Value	122,821	117,851
U.S. Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities available for sale, Amortized Cost	1,042,955	1,019,640
Securities available for sale, Gross Unrealized Gains	6,136	6,597
Securities available for sale, Gross Unrealized Losses	(119)	(122)
Total securities available for sale, Fair Value	1,048,972	1,026,115
Mortgage-backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities available for sale, Amortized Cost	3,218,547	3,480,006
Securities available for sale, Gross Unrealized Gains	57,084	78,600
Securities available for sale, Gross Unrealized Losses	(4,913)	(2,413)
Total securities available for sale, Fair Value	3,270,718	3,556,193
State and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities available for sale, Amortized Cost	1,928,957	1,842,715
Securities available for sale, Gross Unrealized Gains	45,193	51,341
Securities available for sale, Gross Unrealized Losses	(4,214)	(1,372)
Total securities available for sale, Fair Value	1,969,936	1,892,684
Corporate Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities available for sale, Amortized Cost	427,500	337,706
Securities available for sale, Gross Unrealized Gains	2,378	1,945
Securities available for sale, Gross Unrealized Losses	(741)	(764)
Total securities available for sale, Fair Value	429,137	338,887
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities available for sale, Amortized Cost		5,733
Securities available for sale, Gross Unrealized Gains
Securities available for sale, Gross Unrealized Losses
Total securities available for sale, Fair Value		$ 5,733

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

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Reference 2: http://www.xbrl.org/2003/role/presentationRef

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Reference 3: http://www.xbrl.org/2003/role/presentationRef

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Reference 5: http://www.xbrl.org/2003/role/presentationRef

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Reference 6: http://www.xbrl.org/2003/role/presentationRef

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Reference 8: http://www.xbrl.org/2003/role/presentationRef

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Reference 9: http://www.xbrl.org/2003/role/presentationRef

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Reference 10: http://www.xbrl.org/2003/role/presentationRef

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+ Details

- Definition

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.

+ References+ Details

- 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.

+ References+ Details