Investment Securities - Age of Unrealized Losses and Fair Value of Related Securities Held to Maturity (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Less than Twelve Months, Fair Value	$ 635,288	$ 710,652
Less than Twelve Months, Unrealized Losses	(24,090)	(39,085)
More than Twelve Months, Fair Value	86,691	57,806
More than Twelve Months, Unrealized Losses	(13,760)	(12,187)
Total, Fair Value	721,979	768,458
Total, Unrealized Losses	(37,850)	(51,272)
U.S. Treasury Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than Twelve Months, Fair Value		64,537
Less than Twelve Months, Unrealized Losses		(515)
More than Twelve Months, Fair Value		0
More than Twelve Months, Unrealized Losses		0
Total, Fair Value		64,537
Total, Unrealized Losses		(515)
U.S. Government Agency Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than Twelve Months, Fair Value	3,778	4,365
Less than Twelve Months, Unrealized Losses	(13)	(62)
More than Twelve Months, Fair Value	0	0
More than Twelve Months, Unrealized Losses	0	0
Total, Fair Value	3,778	4,365
Total, Unrealized Losses	(13)	(62)
Obligations of States and State Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than Twelve Months, Fair Value	61,791	80,612
Less than Twelve Months, Unrealized Losses	(2,092)	(5,473)
More than Twelve Months, Fair Value	9,968	0
More than Twelve Months, Unrealized Losses	(602)	0
Total, Fair Value	71,759	80,612
Total, Unrealized Losses	(2,694)	(5,473)
Municipal Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than Twelve Months, Fair Value	56,409	85,988
Less than Twelve Months, Unrealized Losses	(2,410)	(5,154)
More than Twelve Months, Fair Value	6,789	1,326
More than Twelve Months, Unrealized Losses	(285)	(105)
Total, Fair Value	63,198	87,314
Total, Unrealized Losses	(2,695)	(5,259)
Obligations of States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than Twelve Months, Fair Value	118,200	166,600
Less than Twelve Months, Unrealized Losses	(4,502)	(10,627)
More than Twelve Months, Fair Value	16,757	1,326
More than Twelve Months, Unrealized Losses	(887)	(105)
Total, Fair Value	134,957	167,926
Total, Unrealized Losses	(5,389)	(10,732)
Residential Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than Twelve Months, Fair Value	498,052	465,400
Less than Twelve Months, Unrealized Losses	(19,333)	(27,631)
More than Twelve Months, Fair Value	13,356	0
More than Twelve Months, Unrealized Losses	(886)	0
Total, Fair Value	511,408	465,400
Total, Unrealized Losses	(20,219)	(27,631)
Trust Preferred Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than Twelve Months, Fair Value	9,783	9,750
Less than Twelve Months, Unrealized Losses	(217)	(250)
More than Twelve Months, Fair Value	56,578	56,480
More than Twelve Months, Unrealized Losses	(11,987)	(12,082)
Total, Fair Value	66,361	66,230
Total, Unrealized Losses	(12,204)	(12,332)
Corporate and Other Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than Twelve Months, Fair Value	5,475
Less than Twelve Months, Unrealized Losses	(25)
More than Twelve Months, Fair Value	0
More than Twelve Months, Unrealized Losses	0
Total, Fair Value	5,475
Total, Unrealized Losses	$ (25)