Investment Securities - Age of Unrealized Losses and Fair Value of Related Securities (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Less than Twelve Months, Fair Value	$ 929,411	$ 100,748
Less than Twelve Months, Unrealized Losses	(6,522)	(999)
More than Twelve Months, Fair Value	241,111	379,752
More than Twelve Months, Unrealized Losses	(9,790)	(11,332)
Total, Fair Value	1,170,522	480,500
Total, Unrealized Losses	(16,312)	(12,331)
U.S. Treasury securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than Twelve Months, Fair Value	548,538	0
Less than Twelve Months, Unrealized Losses	(1,704)	0
More than Twelve Months, Fair Value	0	48,504
More than Twelve Months, Unrealized Losses	0	(1,622)
Total, Fair Value	548,538	48,504
Total, Unrealized Losses	(1,704)	(1,622)
U.S. government agency securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than Twelve Months, Fair Value	3,489	0
Less than Twelve Months, Unrealized Losses	(5)	0
More than Twelve Months, Fair Value	4,736	5,442
More than Twelve Months, Unrealized Losses	(13)	(86)
Total, Fair Value	8,225	5,442
Total, Unrealized Losses	(18)	(86)
Obligations of states and state agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than Twelve Months, Fair Value	24,359	0
Less than Twelve Months, Unrealized Losses	(67)	0
More than Twelve Months, Fair Value	0	11,136
More than Twelve Months, Unrealized Losses	0	(24)
Total, Fair Value	24,359	11,136
Total, Unrealized Losses	(67)	(24)
Municipal bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than Twelve Months, Fair Value	38,207	13,337
Less than Twelve Months, Unrealized Losses	(128)	(426)
More than Twelve Months, Fair Value	13,551	14,637
More than Twelve Months, Unrealized Losses	(246)	(126)
Total, Fair Value	51,758	27,974
Total, Unrealized Losses	(374)	(552)
Obligations of states and political subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than Twelve Months, Fair Value	62,566	13,337
Less than Twelve Months, Unrealized Losses	(195)	(426)
More than Twelve Months, Fair Value	13,551	25,773
More than Twelve Months, Unrealized Losses	(246)	(150)
Total, Fair Value	76,117	39,110
Total, Unrealized Losses	(441)	(576)
Residential mortgage-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than Twelve Months, Fair Value	293,615	57,543
Less than Twelve Months, Unrealized Losses	(4,147)	(121)
More than Twelve Months, Fair Value	164,010	244,910
More than Twelve Months, Unrealized Losses	(4,537)	(4,839)
Total, Fair Value	457,625	302,453
Total, Unrealized Losses	(8,684)	(4,960)
Trust preferred securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than Twelve Months, Fair Value	0	2,210
Less than Twelve Months, Unrealized Losses	0	(117)
More than Twelve Months, Fair Value	8,404	12,085
More than Twelve Months, Unrealized Losses	(2,054)	(2,836)
Total, Fair Value	8,404	14,295
Total, Unrealized Losses	(2,054)	(2,953)
Corporate and other debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than Twelve Months, Fair Value	21,203	27,500
Less than Twelve Months, Unrealized Losses	(471)	(294)
More than Twelve Months, Fair Value	36,137	28,269
More than Twelve Months, Unrealized Losses	(1,418)	(924)
Total, Fair Value	57,340	55,769
Total, Unrealized Losses	(1,889)	(1,218)
Equity securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than Twelve Months, Fair Value	0	158
Less than Twelve Months, Unrealized Losses	0	(41)
More than Twelve Months, Fair Value	14,273	14,769
More than Twelve Months, Unrealized Losses	(1,522)	(875)
Total, Fair Value	14,273	14,927
Total, Unrealized Losses	$ (1,522)	$ (916)