Investment Securities - Age of Unrealized Losses and Fair Value of Related Securities Held to Maturity (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Less than Twelve Months, Fair Value	$ 242,680	$ 112,284
Less than Twelve Months, Unrealized Losses	(1,906)	(613)
More than Twelve Months, Fair Value	217,674	390,639
More than Twelve Months, Unrealized Losses	(16,332)	(18,701)
Total, Fair Value	460,354	502,923
Total, Unrealized Losses	(18,238)	(19,314)
Obligations of states and state agencies
Schedule of Available-for-sale Securities [Line Items]
Less than Twelve Months, Fair Value	0	4,927
Less than Twelve Months, Unrealized Losses	0	(50)
More than Twelve Months, Fair Value	1,957	19,050
More than Twelve Months, Unrealized Losses	(12)	(362)
Total, Fair Value	1,957	23,977
Total, Unrealized Losses	(12)	(412)
Municipal bonds
Schedule of Available-for-sale Securities [Line Items]
Less than Twelve Months, Fair Value	19,027	0
Less than Twelve Months, Unrealized Losses	(241)	0
More than Twelve Months, Fair Value	10,159	28,815
More than Twelve Months, Unrealized Losses	(68)	(278)
Total, Fair Value	29,186	28,815
Total, Unrealized Losses	(309)	(278)
Obligations of states and political subdivisions
Schedule of Available-for-sale Securities [Line Items]
Less than Twelve Months, Fair Value	19,027	4,927
Less than Twelve Months, Unrealized Losses	(241)	(50)
More than Twelve Months, Fair Value	12,116	47,865
More than Twelve Months, Unrealized Losses	(80)	(640)
Total, Fair Value	31,143	52,792
Total, Unrealized Losses	(321)	(690)
Residential mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Less than Twelve Months, Fair Value	223,653	107,357
Less than Twelve Months, Unrealized Losses	(1,665)	(563)
More than Twelve Months, Fair Value	160,149	276,580
More than Twelve Months, Unrealized Losses	(3,235)	(5,681)
Total, Fair Value	383,802	383,937
Total, Unrealized Losses	(4,900)	(6,244)
Trust preferred securities
Schedule of Available-for-sale Securities [Line Items]
Less than Twelve Months, Fair Value	0	0
Less than Twelve Months, Unrealized Losses	0	0
More than Twelve Months, Fair Value	45,409	66,194
More than Twelve Months, Unrealized Losses	(13,017)	(12,380)
Total, Fair Value	45,409	66,194
Total, Unrealized Losses	$ (13,017)	$ (12,380)