Investment Securities - Age of Unrealized Losses and Fair Value of Related Securities (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Less than twelve months, fair value	$ 489,894	$ 942,713
Less than twelve months, unrealized losses	(2,932)	(14,532)
More than twelve months, fair value	691,297	186,670
More than twelve months, unrealized losses	(19,226)	(7,884)
Total, fair value	1,181,191	1,129,383
Total, unrealized losses	(22,158)	(22,416)
U.S. Treasury securities
Schedule of Available-for-sale Securities [Line Items]
Less than twelve months, fair value	916	48,660
Less than twelve months, unrealized losses	(2)	(1,435)
More than twelve months, fair value	48,726	0
More than twelve months, unrealized losses	(1,353)	0
Total, fair value	49,642	48,660
Total, unrealized losses	(1,355)	(1,435)
U.S. government agency securities
Schedule of Available-for-sale Securities [Line Items]
Less than twelve months, fair value	31,177	2,530
Less than twelve months, unrealized losses	(37)	(4)
More than twelve months, fair value	0	4,034
More than twelve months, unrealized losses	0	(2)
Total, fair value	31,177	6,564
Total, unrealized losses	(37)	(6)
Obligations of states and state agencies
Schedule of Available-for-sale Securities [Line Items]
Less than twelve months, fair value	13,337	28,628
Less than twelve months, unrealized losses	(131)	(404)
More than twelve months, fair value	7,792	753
More than twelve months, unrealized losses	(243)	(20)
Total, fair value	21,129	29,381
Total, unrealized losses	(374)	(424)
Municipal bonds
Schedule of Available-for-sale Securities [Line Items]
Less than twelve months, fair value	31,669	42,573
Less than twelve months, unrealized losses	(256)	(506)
More than twelve months, fair value	12,133	11,081
More than twelve months, unrealized losses	(308)	(261)
Total, fair value	43,802	53,654
Total, unrealized losses	(564)	(767)
Total obligations of states and political subdivisions
Schedule of Available-for-sale Securities [Line Items]
Less than twelve months, fair value	45,006	71,201
Less than twelve months, unrealized losses	(387)	(910)
More than twelve months, fair value	19,925	11,834
More than twelve months, unrealized losses	(551)	(281)
Total, fair value	64,931	83,035
Total, unrealized losses	(938)	(1,191)
Residential mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Less than twelve months, fair value	406,940	788,030
Less than twelve months, unrealized losses	(2,461)	(11,889)
More than twelve months, fair value	599,167	132,718
More than twelve months, unrealized losses	(16,201)	(4,457)
Total, fair value	1,006,107	920,748
Total, unrealized losses	(18,662)	(16,346)
Trust preferred securities
Schedule of Available-for-sale Securities [Line Items]
Less than twelve months, fair value	0
Less than twelve months, unrealized losses	0
More than twelve months, fair value	3,214	8,009
More than twelve months, unrealized losses	(512)	(2,155)
Total, fair value	3,214	8,009
Total, unrealized losses	(512)	(2,155)
Corporate and other debt securities
Schedule of Available-for-sale Securities [Line Items]
Less than twelve months, fair value	5,855	32,292
Less than twelve months, unrealized losses	(45)	(294)
More than twelve months, fair value	15,115	15,192
More than twelve months, unrealized losses	(115)	(228)
Total, fair value	20,970	47,484
Total, unrealized losses	(160)	(522)
Equity 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	5,150	14,883
More than twelve months, unrealized losses	(494)	(761)
Total, fair value	5,150	14,883
Total, unrealized losses	$ (494)	$ (761)