Investment Securities - Age of Unrealized Losses and Fair Value of Related Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities
Less than Twelve Months, Fair Value	$ 169,052	$ 929,411
Less than Twelve Months, Unrealized Losses	(613)	(6,522)
More than Twelve Months, Fair Value	192,116	241,111
More than Twelve Months, Unrealized Losses	(5,744)	(9,790)
Total, Fair Value	361,168	1,170,522
Total, Unrealized Losses	(6,357)	(16,312)
U.S. Treasury securities
Schedule of Available-for-sale Securities
Less than Twelve Months, Fair Value		548,538
Less than Twelve Months, Unrealized Losses		(1,704)
More than Twelve Months, Fair Value		0
More than Twelve Months, Unrealized Losses		0
Total, Fair Value		548,538
Total, Unrealized Losses		(1,704)
U.S. government agency securities
Schedule of Available-for-sale Securities
Less than Twelve Months, Fair Value	0	3,489
Less than Twelve Months, Unrealized Losses	0	(5)
More than Twelve Months, Fair Value	4,197	4,736
More than Twelve Months, Unrealized Losses	(6)	(13)
Total, Fair Value	4,197	8,225
Total, Unrealized Losses	(6)	(18)
Obligations of states and state agencies
Schedule of Available-for-sale Securities
Less than Twelve Months, Fair Value	2,607	24,359
Less than Twelve Months, Unrealized Losses	(40)	(67)
More than Twelve Months, Fair Value	0	0
More than Twelve Months, Unrealized Losses	0	0
Total, Fair Value	2,607	24,359
Total, Unrealized Losses	(40)	(67)
Municipal bonds
Schedule of Available-for-sale Securities
Less than Twelve Months, Fair Value	0	38,207
Less than Twelve Months, Unrealized Losses	0	(128)
More than Twelve Months, Fair Value	11,012	13,551
More than Twelve Months, Unrealized Losses	(321)	(246)
Total, Fair Value	11,012	51,758
Total, Unrealized Losses	(321)	(374)
Obligations of states and political subdivisions
Schedule of Available-for-sale Securities
Less than Twelve Months, Fair Value	2,607	62,566
Less than Twelve Months, Unrealized Losses	(40)	(195)
More than Twelve Months, Fair Value	11,012	13,551
More than Twelve Months, Unrealized Losses	(321)	(246)
Total, Fair Value	13,619	76,117
Total, Unrealized Losses	(361)	(441)
Residential mortgage-backed securities
Schedule of Available-for-sale Securities
Less than Twelve Months, Fair Value	145,631	293,615
Less than Twelve Months, Unrealized Losses	(566)	(4,147)
More than Twelve Months, Fair Value	138,425	164,010
More than Twelve Months, Unrealized Losses	(2,348)	(4,537)
Total, Fair Value	284,056	457,625
Total, Unrealized Losses	(2,914)	(8,684)
Trust preferred securities
Schedule of Available-for-sale Securities
Less than Twelve Months, Fair Value	0	0
Less than Twelve Months, Unrealized Losses	0	0
More than Twelve Months, Fair Value	8,155	8,404
More than Twelve Months, Unrealized Losses	(2,075)	(2,054)
Total, Fair Value	8,155	8,404
Total, Unrealized Losses	(2,075)	(2,054)
Corporate and other debt securities
Schedule of Available-for-sale Securities
Less than Twelve Months, Fair Value	20,814	21,203
Less than Twelve Months, Unrealized Losses	(7)	(471)
More than Twelve Months, Fair Value	15,329	36,137
More than Twelve Months, Unrealized Losses	(197)	(1,418)
Total, Fair Value	36,143	57,340
Total, Unrealized Losses	(204)	(1,889)
Equity securities
Schedule of Available-for-sale Securities
Less than Twelve Months, Fair Value	0	0
Less than Twelve Months, Unrealized Losses	0	0
More than Twelve Months, Fair Value	14,998	14,273
More than Twelve Months, Unrealized Losses	(797)	(1,522)
Total, Fair Value	14,998	14,273
Total, Unrealized Losses	$ (797)	$ (1,522)