Investment Securities - Age of Unrealized Losses and Fair Value of Related Securities (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Less than Twelve Months, Fair Value	$ 942,713	$ 929,411
Less than Twelve Months, Unrealized Losses	(14,532)	(6,522)
More than Twelve Months, Fair Value	186,670	241,111
More than Twelve Months, Unrealized Losses	(7,884)	(9,790)
Total, Fair Value	1,129,383	1,170,522
Total, Unrealized Losses	(22,416)	(16,312)
U.S. Treasury securities
Schedule of Available-for-sale Securities [Line Items]
Less than Twelve Months, Fair Value	48,660	548,538
Less than Twelve Months, Unrealized Losses	(1,435)	(1,704)
More than Twelve Months, Fair Value	0	0
More than Twelve Months, Unrealized Losses	0	0
Total, Fair Value	48,660	548,538
Total, Unrealized Losses	(1,435)	(1,704)
U.S. government agency securities
Schedule of Available-for-sale Securities [Line Items]
Less than Twelve Months, Fair Value	2,530	3,489
Less than Twelve Months, Unrealized Losses	(4)	(5)
More than Twelve Months, Fair Value	4,034	4,736
More than Twelve Months, Unrealized Losses	(2)	(13)
Total, Fair Value	6,564	8,225
Total, Unrealized Losses	(6)	(18)
Obligations of states and state agencies
Schedule of Available-for-sale Securities [Line Items]
Less than Twelve Months, Fair Value	28,628	24,359
Less than Twelve Months, Unrealized Losses	(404)	(67)
More than Twelve Months, Fair Value	753	0
More than Twelve Months, Unrealized Losses	(20)	0
Total, Fair Value	29,381	24,359
Total, Unrealized Losses	(424)	(67)
Municipal bonds
Schedule of Available-for-sale Securities [Line Items]
Less than Twelve Months, Fair Value	42,573	38,207
Less than Twelve Months, Unrealized Losses	(506)	(128)
More than Twelve Months, Fair Value	11,081	13,551
More than Twelve Months, Unrealized Losses	(261)	(246)
Total, Fair Value	53,654	51,758
Total, Unrealized Losses	(767)	(374)
Total obligations of states and political subdivisions
Schedule of Available-for-sale Securities [Line Items]
Less than Twelve Months, Fair Value	71,201	62,566
Less than Twelve Months, Unrealized Losses	(910)	(195)
More than Twelve Months, Fair Value	11,834	13,551
More than Twelve Months, Unrealized Losses	(281)	(246)
Total, Fair Value	83,035	76,117
Total, Unrealized Losses	(1,191)	(441)
Residential mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Less than Twelve Months, Fair Value	788,030	293,615
Less than Twelve Months, Unrealized Losses	(11,889)	(4,147)
More than Twelve Months, Fair Value	132,718	164,010
More than Twelve Months, Unrealized Losses	(4,457)	(4,537)
Total, Fair Value	920,748	457,625
Total, Unrealized Losses	(16,346)	(8,684)
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	8,009	8,404
More than Twelve Months, Unrealized Losses	(2,155)	(2,054)
Total, Fair Value	8,009	8,404
Total, Unrealized Losses	(2,155)	(2,054)
Corporate and other debt securities
Schedule of Available-for-sale Securities [Line Items]
Less than Twelve Months, Fair Value	32,292	21,203
Less than Twelve Months, Unrealized Losses	(294)	(471)
More than Twelve Months, Fair Value	15,192	36,137
More than Twelve Months, Unrealized Losses	(228)	(1,418)
Total, Fair Value	47,484	57,340
Total, Unrealized Losses	(522)	(1,889)
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	14,883	14,273
More than Twelve Months, Unrealized Losses	(761)	(1,522)
Total, Fair Value	14,883	14,273
Total, Unrealized Losses	$ (761)	$ (1,522)