Investment Securities - Unrealized Losses and Fair Value Available for Sale (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Fair Value
Less than Twelve Months	$ 1,040,852	$ 489,894
More than Twelve Months	641,197	691,297
Total	1,682,049	1,181,191
Unrealized Losses
Less than Twelve Months	(21,209)	(2,932)
More than Twelve Months	(28,152)	(19,226)
Total	(49,361)	(22,158)
U.S. Treasury securities
Fair Value
Less than Twelve Months	906	916
More than Twelve Months	48,000	48,726
Total	48,906	49,642
Unrealized Losses
Less than Twelve Months	(11)	(2)
More than Twelve Months	(2,075)	(1,353)
Total	(2,086)	(1,355)
U.S. government agency securities
Fair Value
Less than Twelve Months	32,584	31,177
More than Twelve Months	0	0
Total	32,584	31,177
Unrealized Losses
Less than Twelve Months	(682)	(37)
More than Twelve Months	0	0
Total	(682)	(37)
Obligations of states and state agencies
Fair Value
Less than Twelve Months	100,090	13,337
More than Twelve Months	7,674	7,792
Total	107,764	21,129
Unrealized Losses
Less than Twelve Months	(1,557)	(131)
More than Twelve Months	(296)	(243)
Total	(1,853)	(374)
Municipal bonds
Fair Value
Less than Twelve Months	71,192	31,669
More than Twelve Months	12,067	12,133
Total	83,259	43,802
Unrealized Losses
Less than Twelve Months	(937)	(256)
More than Twelve Months	(369)	(308)
Total	(1,306)	(564)
Obligations of states and political subdivisions
Fair Value
Less than Twelve Months	171,282	45,006
More than Twelve Months	19,741	19,925
Total	191,023	64,931
Unrealized Losses
Less than Twelve Months	(2,494)	(387)
More than Twelve Months	(665)	(551)
Total	(3,159)	(938)
Residential mortgage-backed securities
Fair Value
Less than Twelve Months	811,654	406,940
More than Twelve Months	565,498	599,167
Total	1,377,152	1,006,107
Unrealized Losses
Less than Twelve Months	(17,738)	(2,461)
More than Twelve Months	(24,757)	(16,201)
Total	(42,495)	(18,662)
Trust preferred securities
Fair Value
Less than Twelve Months	0	0
More than Twelve Months	3,183	3,214
Total	3,183	3,214
Unrealized Losses
Less than Twelve Months	0	0
More than Twelve Months	(445)	(512)
Total	(445)	(512)
Corporate and other debt securities
Fair Value
Less than Twelve Months	24,426	5,855
More than Twelve Months	4,775	15,115
Total	29,201	20,970
Unrealized Losses
Less than Twelve Months	(284)	(45)
More than Twelve Months	(210)	(115)
Total	$ (494)	(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)