Investment Securities - Age of Unrealized Losses and Fair Value of Related Securities Held to Maturity (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Schedule of Held-to-Maturity Securities [Line Items]
Less than Twelve Months, Fair Value	$ 263,042	$ 112,284
Less than Twelve Months, Unrealized Losses	(3,042)	(613)
More than Twelve Months, Fair Value	219,966	390,639
More than Twelve Months, Unrealized Losses	(18,521)	(18,701)
Total, Fair Value	483,008	502,923
Total, Unrealized Losses	(21,563)	(19,314)
Obligations of states and state agencies [Member]
Schedule of Held-to-Maturity Securities [Line Items]
Less than Twelve Months, Fair Value	6,837	4,927
Less than Twelve Months, Unrealized Losses	(5)	(50)
More than Twelve Months, Fair Value	1,965	19,050
More than Twelve Months, Unrealized Losses	(5)	(362)
Total, Fair Value	8,802	23,977
Total, Unrealized Losses	(10)	(412)
Municipal bonds [Member]
Schedule of Held-to-Maturity Securities [Line Items]
Less than Twelve Months, Fair Value	8,814	0
Less than Twelve Months, Unrealized Losses	(72)	0
More than Twelve Months, Fair Value	10,198	28,815
More than Twelve Months, Unrealized Losses	(13)	(278)
Total, Fair Value	19,012	28,815
Total, Unrealized Losses	(85)	(278)
Obligations of states and political subdivisions [Member]
Schedule of Held-to-Maturity Securities [Line Items]
Less than Twelve Months, Fair Value	15,651	4,927
Less than Twelve Months, Unrealized Losses	(77)	(50)
More than Twelve Months, Fair Value	12,163	47,865
More than Twelve Months, Unrealized Losses	(18)	(640)
Total, Fair Value	27,814	52,792
Total, Unrealized Losses	(95)	(690)
Residential mortgage-backed securities [Member]
Schedule of Held-to-Maturity Securities [Line Items]
Less than Twelve Months, Fair Value	244,440	107,357
Less than Twelve Months, Unrealized Losses	(2,916)	(563)
More than Twelve Months, Fair Value	162,756	276,580
More than Twelve Months, Unrealized Losses	(5,119)	(5,681)
Total, Fair Value	407,196	383,937
Total, Unrealized Losses	(8,035)	(6,244)
Trust preferred securities [Member]
Schedule of Held-to-Maturity Securities [Line Items]
Less than Twelve Months, Fair Value	0	0
Less than Twelve Months, Unrealized Losses	0	0
More than Twelve Months, Fair Value	45,047	66,194
More than Twelve Months, Unrealized Losses	(13,384)	(12,380)
Total, Fair Value	45,047	66,194
Total, Unrealized Losses	(13,384)	$ (12,380)
Corporate and other debt securities [Member]
Schedule of Held-to-Maturity Securities [Line Items]
Less than Twelve Months, Fair Value	2,951
Less than Twelve Months, Unrealized Losses	(49)
More than Twelve Months, Fair Value	0
More than Twelve Months, Unrealized Losses	0
Total, Fair Value	2,951
Total, Unrealized Losses	$ (49)