Investment Securities - Unrealized Losses and Fair Value Available for Sale (Detail) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Fair Value
Less than Twelve Months	$ 881,521	$ 942,713
More than Twelve Months	175,334	186,670
Total	1,056,855	1,129,383
Unrealized Losses
Less than Twelve Months	(11,104)	(14,532)
More than Twelve Months	(6,853)	(7,884)
Total	(17,957)	(22,416)
U.S. Treasury securities
Fair Value
Less than Twelve Months	49,168	48,660
More than Twelve Months	0	0
Total	49,168	48,660
Unrealized Losses
Less than Twelve Months	(919)	(1,435)
More than Twelve Months	0	0
Total	(919)	(1,435)
U.S. government agency securities
Fair Value
Less than Twelve Months	31,236	2,530
More than Twelve Months	3,808	4,034
Total	35,044	6,564
Unrealized Losses
Less than Twelve Months	(68)	(4)
More than Twelve Months	(3)	(2)
Total	(71)	(6)
Obligations of states and state agencies
Fair Value
Less than Twelve Months	13,118	28,628
More than Twelve Months	1,628	753
Total	14,746	29,381
Unrealized Losses
Less than Twelve Months	(169)	(404)
More than Twelve Months	(49)	(20)
Total	(218)	(424)
Municipal bonds
Fair Value
Less than Twelve Months	18,302	42,573
More than Twelve Months	11,059	11,081
Total	29,361	53,654
Unrealized Losses
Less than Twelve Months	(193)	(506)
More than Twelve Months	(300)	(261)
Total	(493)	(767)
Obligations of states and political subdivisions
Fair Value
Less than Twelve Months	31,420	71,201
More than Twelve Months	12,687	11,834
Total	44,107	83,035
Unrealized Losses
Less than Twelve Months	(362)	(910)
More than Twelve Months	(349)	(281)
Total	(711)	(1,191)
Residential mortgage-backed securities
Fair Value
Less than Twelve Months	739,362	788,030
More than Twelve Months	136,402	132,718
Total	875,764	920,748
Unrealized Losses
Less than Twelve Months	(9,680)	(11,889)
More than Twelve Months	(4,335)	(4,457)
Total	(14,015)	(16,346)
Trust preferred securities
Fair Value
Less than Twelve Months	0	0
More than Twelve Months	6,219	8,009
Total	6,219	8,009
Unrealized Losses
Less than Twelve Months	0	0
More than Twelve Months	(1,577)	(2,155)
Total	(1,577)	(2,155)
Corporate and other debt securities
Fair Value
Less than Twelve Months	30,335	32,292
More than Twelve Months	11,034	15,192
Total	41,369	47,484
Unrealized Losses
Less than Twelve Months	(75)	(294)
More than Twelve Months	(129)	(228)
Total	(204)	(522)
Equity securities
Fair Value
Less than Twelve Months	0	0
More than Twelve Months	5,184	14,883
Total	5,184	14,883
Unrealized Losses
Less than Twelve Months	0	0
More than Twelve Months	(460)	(761)
Total	$ (460)	$ (761)