Investment Securities - Unrealized Losses and Fair Value Available for Sale (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Fair Value
Less than Twelve Months	$ 699,848	$ 942,713
More than Twelve Months	255,694	186,670
Total	955,542	1,129,383
Unrealized Losses
Less than Twelve Months	(9,003)	(14,532)
More than Twelve Months	(7,898)	(7,884)
Total	(16,901)	(22,416)
U.S. Treasury securities
Fair Value
Less than Twelve Months	49,223	48,660
More than Twelve Months	0	0
Total	49,223	48,660
Unrealized Losses
Less than Twelve Months	(861)	(1,435)
More than Twelve Months	0	0
Total	(861)	(1,435)
U.S. government agency securities
Fair Value
Less than Twelve Months	5,527	2,530
More than Twelve Months	0	4,034
Total	5,527	6,564
Unrealized Losses
Less than Twelve Months	(8)	(4)
More than Twelve Months	0	(2)
Total	(8)	(6)
Obligations of states and state agencies
Fair Value
Less than Twelve Months	11,117	28,628
More than Twelve Months	1,544	753
Total	12,661	29,381
Unrealized Losses
Less than Twelve Months	(153)	(404)
More than Twelve Months	(51)	(20)
Total	(204)	(424)
Municipal bonds
Fair Value
Less than Twelve Months	18,688	42,573
More than Twelve Months	11,019	11,081
Total	29,707	53,654
Unrealized Losses
Less than Twelve Months	(184)	(506)
More than Twelve Months	(349)	(261)
Total	(533)	(767)
Obligations of states and political subdivisions
Fair Value
Less than Twelve Months	29,805	71,201
More than Twelve Months	12,563	11,834
Total	42,368	83,035
Unrealized Losses
Less than Twelve Months	(337)	(910)
More than Twelve Months	(400)	(281)
Total	(737)	(1,191)
Residential mortgage-backed securities
Fair Value
Less than Twelve Months	599,413	788,030
More than Twelve Months	217,306	132,718
Total	816,719	920,748
Unrealized Losses
Less than Twelve Months	(7,780)	(11,889)
More than Twelve Months	(5,803)	(4,457)
Total	(13,583)	(16,346)
Trust preferred securities
Fair Value
Less than Twelve Months	0	0
More than Twelve Months	5,394	8,009
Total	5,394	8,009
Unrealized Losses
Less than Twelve Months	0	0
More than Twelve Months	(1,139)	(2,155)
Total	(1,139)	(2,155)
Corporate and other debt securities
Fair Value
Less than Twelve Months	15,880	32,292
More than Twelve Months	15,241	15,192
Total	31,121	47,484
Unrealized Losses
Less than Twelve Months	(17)	(294)
More than Twelve Months	(102)	(228)
Total	(119)	(522)
Equity securities
Fair Value
Less than Twelve Months	0	0
More than Twelve Months	5,190	14,883
Total	5,190	14,883
Unrealized Losses
Less than Twelve Months	0	0
More than Twelve Months	(454)	(761)
Total	$ (454)	$ (761)