Investment Securities - Unrealized Losses and Fair Value Available for Sale (Detail) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities
Less than Twelve Months, Fair Value	$ 884,055	$ 942,713
Less than Twelve Months, Unrealized Losses	(13,446)	(14,532)
More than Twelve Months, Fair Value	183,504	186,670
More than Twelve Months, Unrealized Losses	(7,326)	(7,884)
Total, Fair Value	1,067,559	1,129,383
Total, Unrealized Losses	(20,772)	(22,416)
U.S. Treasury securities
Schedule of Available-for-sale Securities
Less than Twelve Months, Fair Value	48,852	48,660
Less than Twelve Months, Unrealized Losses	(1,240)	(1,435)
More than Twelve Months, Fair Value	0	0
More than Twelve Months, Unrealized Losses	0	0
Total, Fair Value	48,852	48,660
Total, Unrealized Losses	(1,240)	(1,435)
U.S. government agency securities
Schedule of Available-for-sale Securities
Less than Twelve Months, Fair Value	29,314	2,530
Less than Twelve Months, Unrealized Losses	(18)	(4)
More than Twelve Months, Fair Value	3,940	4,034
More than Twelve Months, Unrealized Losses	(3)	(2)
Total, Fair Value	33,254	6,564
Total, Unrealized Losses	(21)	(6)
Obligations of states and state agencies
Schedule of Available-for-sale Securities
Less than Twelve Months, Fair Value	16,778	28,628
Less than Twelve Months, Unrealized Losses	(203)	(404)
More than Twelve Months, Fair Value	0	753
More than Twelve Months, Unrealized Losses	0	(20)
Total, Fair Value	16,778	29,381
Total, Unrealized Losses	(203)	(424)
Municipal bonds
Schedule of Available-for-sale Securities
Less than Twelve Months, Fair Value	29,738	42,573
Less than Twelve Months, Unrealized Losses	(314)	(506)
More than Twelve Months, Fair Value	11,136	11,081
More than Twelve Months, Unrealized Losses	(214)	(261)
Total, Fair Value	40,874	53,654
Total, Unrealized Losses	(528)	(767)
Obligations of states and political subdivisions
Schedule of Available-for-sale Securities
Less than Twelve Months, Fair Value	46,516	71,201
Less than Twelve Months, Unrealized Losses	(517)	(910)
More than Twelve Months, Fair Value	11,136	11,834
More than Twelve Months, Unrealized Losses	(214)	(281)
Total, Fair Value	57,652	83,035
Total, Unrealized Losses	(731)	(1,191)
Residential mortgage-backed securities
Schedule of Available-for-sale Securities
Less than Twelve Months, Fair Value	732,989	788,030
Less than Twelve Months, Unrealized Losses	(11,579)	(11,889)
More than Twelve Months, Fair Value	141,743	132,718
More than Twelve Months, Unrealized Losses	(4,988)	(4,457)
Total, Fair Value	874,732	920,748
Total, Unrealized Losses	(16,567)	(16,346)
Trust preferred securities
Schedule of Available-for-sale Securities
Less than Twelve Months, Fair Value	0
Less than Twelve Months, Unrealized Losses	0
More than Twelve Months, Fair Value	6,393	8,009
More than Twelve Months, Unrealized Losses	(1,455)	(2,155)
Total, Fair Value	6,393	8,009
Total, Unrealized Losses	(1,455)	(2,155)
Corporate and other debt securities
Schedule of Available-for-sale Securities
Less than Twelve Months, Fair Value	26,384	32,292
Less than Twelve Months, Unrealized Losses	(92)	(294)
More than Twelve Months, Fair Value	15,113	15,192
More than Twelve Months, Unrealized Losses	(201)	(228)
Total, Fair Value	41,497	47,484
Total, Unrealized Losses	(293)	(522)
Equity securities
Schedule of Available-for-sale Securities
Less than Twelve Months, Fair Value	0	0
Less than Twelve Months, Unrealized Losses	0	0
More than Twelve Months, Fair Value	5,179	14,883
More than Twelve Months, Unrealized Losses	(465)	(761)
Total, Fair Value	5,179	14,883
Total, Unrealized Losses	$ (465)	$ (761)