Investment Securities - Age of Unrealized Losses and Fair Value of Related Securities (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Fair Value
Less than Twelve Months	$ 156,682	$ 489,894
More than Twelve Months	1,456,424	691,297
Total	1,613,106	1,181,191
Unrealized Losses
Less than Twelve Months	(1,050)	(2,932)
More than Twelve Months	(47,805)	(19,226)
Total	(48,855)	(22,158)
U.S. Treasury securities
Fair Value
Less than Twelve Months	0	916
More than Twelve Months	49,306	48,726
Total	49,306	49,642
Unrealized Losses
Less than Twelve Months	0	(2)
More than Twelve Months	(1,669)	(1,353)
Total	(1,669)	(1,355)
U.S. government agency securities
Fair Value
Less than Twelve Months	2,120	31,177
More than Twelve Months	26,775	0
Total	28,895	31,177
Unrealized Losses
Less than Twelve Months	(20)	(37)
More than Twelve Months	(618)	0
Total	(638)	(37)
Obligations of states and state agencies
Fair Value
Less than Twelve Months	17,560	13,337
More than Twelve Months	75,718	7,792
Total	93,278	21,129
Unrealized Losses
Less than Twelve Months	(95)	(131)
More than Twelve Months	(3,587)	(243)
Total	(3,682)	(374)
Municipal bonds
Fair Value
Less than Twelve Months	5,018	31,669
More than Twelve Months	70,286	12,133
Total	75,304	43,802
Unrealized Losses
Less than Twelve Months	(106)	(256)
More than Twelve Months	(1,331)	(308)
Total	(1,437)	(564)
Total obligations of states and political subdivisions
Fair Value
Less than Twelve Months	22,578	45,006
More than Twelve Months	146,004	19,925
Total	168,582	64,931
Unrealized Losses
Less than Twelve Months	(201)	(387)
More than Twelve Months	(4,918)	(551)
Total	(5,119)	(938)
Residential mortgage-backed securities
Fair Value
Less than Twelve Months	119,645	406,940
More than Twelve Months	1,221,942	599,167
Total	1,341,587	1,006,107
Unrealized Losses
Less than Twelve Months	(668)	(2,461)
More than Twelve Months	(40,093)	(16,201)
Total	(40,761)	(18,662)
Trust preferred securities
Fair Value
Less than Twelve Months		0
More than Twelve Months		3,214
Total		3,214
Unrealized Losses
Less than Twelve Months		0
More than Twelve Months		(512)
Total		(512)
Corporate and other debt securities
Fair Value
Less than Twelve Months	12,339	5,855
More than Twelve Months	12,397	15,115
Total	24,736	20,970
Unrealized Losses
Less than Twelve Months	(161)	(45)
More than Twelve Months	(507)	(115)
Total	$ (668)	(160)
Equity securities
Fair Value
Less than Twelve Months		0
More than Twelve Months		5,150
Total		5,150
Unrealized Losses
Less than Twelve Months		0
More than Twelve Months		(494)
Total		$ (494)