Investment Securities - Age of Unrealized Losses and Fair Value of Related Securities (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Less than Twelve Months, Fair Value	$ 100,748	$ 559,333
Less than Twelve Months, Unrealized Losses	(999)	(38,574)
More than Twelve Months, Fair Value	379,752	56,475
More than Twelve Months, Unrealized Losses	(11,332)	(7,688)
Total, Fair Value	480,500	615,808
Total, Unrealized Losses	(12,331)	(46,262)
U.S. Treasury securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than Twelve Months, Fair Value	0	84,665
Less than Twelve Months, Unrealized Losses	0	(15,170)
More than Twelve Months, Fair Value	48,504	0
More than Twelve Months, Unrealized Losses	(1,622)	0
Total, Fair Value	48,504	84,665
Total, Unrealized Losses	(1,622)	(15,170)
U.S. government agency securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than Twelve Months, Fair Value	0	26,402
Less than Twelve Months, Unrealized Losses	0	(703)
More than Twelve Months, Fair Value	5,442	0
More than Twelve Months, Unrealized Losses	(86)	0
Total, Fair Value	5,442	26,402
Total, Unrealized Losses	(86)	(703)
Obligations of states and state agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than Twelve Months, Fair Value	0	10,598
Less than Twelve Months, Unrealized Losses	0	(798)
More than Twelve Months, Fair Value	11,136	0
More than Twelve Months, Unrealized Losses	(24)	0
Total, Fair Value	11,136	10,598
Total, Unrealized Losses	(24)	(798)
Municipal bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than Twelve Months, Fair Value	13,337	13,461
Less than Twelve Months, Unrealized Losses	(426)	(1,365)
More than Twelve Months, Fair Value	14,637	0
More than Twelve Months, Unrealized Losses	(126)	0
Total, Fair Value	27,974	13,461
Total, Unrealized Losses	(552)	(1,365)
Obligations of states and political subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than Twelve Months, Fair Value	13,337	24,059
Less than Twelve Months, Unrealized Losses	(426)	(2,163)
More than Twelve Months, Fair Value	25,773	0
More than Twelve Months, Unrealized Losses	(150)	0
Total, Fair Value	39,110	24,059
Total, Unrealized Losses	(576)	(2,163)
Residential mortgage-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than Twelve Months, Fair Value	57,543	368,306
Less than Twelve Months, Unrealized Losses	(121)	(18,434)
More than Twelve Months, Fair Value	244,910	24,734
More than Twelve Months, Unrealized Losses	(4,839)	(2,285)
Total, Fair Value	302,453	393,040
Total, Unrealized Losses	(4,960)	(20,719)
Trust preferred securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than Twelve Months, Fair Value	2,210	2,024
Less than Twelve Months, Unrealized Losses	(117)	(25)
More than Twelve Months, Fair Value	12,085	15,022
More than Twelve Months, Unrealized Losses	(2,836)	(4,206)
Total, Fair Value	14,295	17,046
Total, Unrealized Losses	(2,953)	(4,231)
Corporate and other debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than Twelve Months, Fair Value	27,500	53,654
Less than Twelve Months, Unrealized Losses	(294)	(2,073)
More than Twelve Months, Fair Value	28,269	2,471
More than Twelve Months, Unrealized Losses	(924)	(30)
Total, Fair Value	55,769	56,125
Total, Unrealized Losses	(1,218)	(2,103)
Equity securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than Twelve Months, Fair Value	158	223
Less than Twelve Months, Unrealized Losses	(41)	(6)
More than Twelve Months, Fair Value	14,769	14,248
More than Twelve Months, Unrealized Losses	(875)	(1,167)
Total, Fair Value	14,927	14,471
Total, Unrealized Losses	$ (916)	$ (1,173)