Gross Unrealized Losses and Fair Value of Investment Securities Available for Sale (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Gain (Loss) on Investments [Line Items]
Less than 12 months, Fair Value	$ 1,243,194	$ 1,041,126
Less than 12 months, Unrealized Losses	(10,149)	(4,828)
12 months or more, Fair Value	5,540	2,604
12 months or more, Unrealized Losses	(54)	(14)
Total Fair Value	1,248,734	1,043,730
Total Unrealized Losses	(10,203)	(4,842)
U.S. Treasury [Member]
Gain (Loss) on Investments [Line Items]
Less than 12 months, Fair Value	19,765	29,747
Less than 12 months, Unrealized Losses	(216)	(171)
12 months or more, Fair Value
12 months or more, Unrealized Losses
Total Fair Value	19,765	29,747
Total Unrealized Losses	(216)	(171)
U.S. Agencies [Member]
Gain (Loss) on Investments [Line Items]
Less than 12 months, Fair Value	195,815	295,747
Less than 12 months, Unrealized Losses	(119)	(122)
12 months or more, Fair Value
12 months or more, Unrealized Losses
Total Fair Value	195,815	295,747
Total Unrealized Losses	(119)	(122)
Mortgage-backed [Member]
Gain (Loss) on Investments [Line Items]
Less than 12 months, Fair Value	583,894	398,384
Less than 12 months, Unrealized Losses	(4,913)	(2,413)
12 months or more, Fair Value
12 months or more, Unrealized Losses
Total Fair Value	583,894	398,384
Total Unrealized Losses	(4,913)	(2,413)
State and Political Subdivisions [Member]
Gain (Loss) on Investments [Line Items]
Less than 12 months, Fair Value	289,738	132,951
Less than 12 months, Unrealized Losses	(4,160)	(1,358)
12 months or more, Fair Value	5,540	2,604
12 months or more, Unrealized Losses	(54)	(14)
Total Fair Value	295,278	135,555
Total Unrealized Losses	(4,214)	(1,372)
Corporate Securities [Member]
Gain (Loss) on Investments [Line Items]
Less than 12 months, Fair Value	153,982	178,564
Less than 12 months, Unrealized Losses	(741)	(764)
12 months or more, Fair Value
12 months or more, Unrealized Losses
Total Fair Value	153,982	178,564
Total Unrealized Losses	(741)	(764)
Commercial Paper [Member]
Gain (Loss) on Investments [Line Items]
Less than 12 months, Fair Value		5,733
Less than 12 months, Unrealized Losses
12 months or more, Fair Value
12 months or more, Unrealized Losses
Total Fair Value		5,733
Total Unrealized Losses