Securities Available for Sale (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Schedule of Available-for-sale Securities [Line Items]
Securities available for sale, Amortized Cost	$ 6,326,760	$ 5,979,906
Securities available for sale, Gross Unrealized Gains	160,916	129,123
Securities available for sale, Gross Unrealized Losses	(552)	(1,147)
Total securities available for sale, Fair Value	6,487,124	6,107,882
U.S. Treasury
Schedule of Available-for-sale Securities [Line Items]
Securities available for sale, Amortized Cost	117,064	184,523
Securities available for sale, Gross Unrealized Gains	1,489	4,802
Securities available for sale, Gross Unrealized Losses	(36)
Total securities available for sale, Fair Value	118,517	189,325
U.S. Agencies
Schedule of Available-for-sale Securities [Line Items]
Securities available for sale, Amortized Cost	809,490	1,615,637
Securities available for sale, Gross Unrealized Gains	8,047	16,434
Securities available for sale, Gross Unrealized Losses	(13)	(62)
Total securities available for sale, Fair Value	817,524	1,632,009
Mortgage-backed
Schedule of Available-for-sale Securities [Line Items]
Securities available for sale, Amortized Cost	3,344,828	2,437,282
Securities available for sale, Gross Unrealized Gains	91,997	55,985
Securities available for sale, Gross Unrealized Losses	(202)	(919)
Total securities available for sale, Fair Value	3,436,623	2,492,348
State and political subdivisions
Schedule of Available-for-sale Securities [Line Items]
Securities available for sale, Amortized Cost	1,827,424	1,642,844
Securities available for sale, Gross Unrealized Gains	56,572	51,336
Securities available for sale, Gross Unrealized Losses	(269)	(144)
Total securities available for sale, Fair Value	1,883,727	1,694,036
Corporates
Schedule of Available-for-sale Securities [Line Items]
Securities available for sale, Amortized Cost	227,954	99,620
Securities available for sale, Gross Unrealized Gains	2,811	566
Securities available for sale, Gross Unrealized Losses	(32)	(22)
Total securities available for sale, Fair Value	$ 230,733	$ 100,164

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

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+ 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