Investment Securities - Age of Unrealized Losses and Fair Value of Related Securities Held to Maturity (Detail) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Less than Twelve Months, Fair Value	$ 174,784	$ 263,042
Less than Twelve Months, Unrealized Losses	(1,010)	(3,042)
More than Twelve Months, Fair Value	263,385	219,966
More than Twelve Months, Unrealized Losses	(16,086)	(18,521)
Total, Fair Value	438,169	483,008
Total, Unrealized Losses	(17,096)	(21,563)
Obligations of states and state agencies
Schedule of Available-for-sale Securities [Line Items]
Less than Twelve Months, Fair Value		6,837
Less than Twelve Months, Unrealized Losses		(5)
More than Twelve Months, Fair Value		1,965
More than Twelve Months, Unrealized Losses		(5)
Total, Fair Value		8,802
Total, Unrealized Losses		(10)
Municipal bonds
Schedule of Available-for-sale Securities [Line Items]
Less than Twelve Months, Fair Value		8,814
Less than Twelve Months, Unrealized Losses		(72)
More than Twelve Months, Fair Value		10,198
More than Twelve Months, Unrealized Losses		(13)
Total, Fair Value		19,012
Total, Unrealized Losses		(85)
Obligations of states and political subdivisions
Schedule of Available-for-sale Securities [Line Items]
Less than Twelve Months, Fair Value		15,651
Less than Twelve Months, Unrealized Losses		(77)
More than Twelve Months, Fair Value		12,163
More than Twelve Months, Unrealized Losses		(18)
Total, Fair Value		27,814
Total, Unrealized Losses		(95)
Residential mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Less than Twelve Months, Fair Value	174,784	244,440
Less than Twelve Months, Unrealized Losses	(1,010)	(2,916)
More than Twelve Months, Fair Value	218,383	162,756
More than Twelve Months, Unrealized Losses	(2,652)	(5,119)
Total, Fair Value	393,167	407,196
Total, Unrealized Losses	(3,662)	(8,035)
Trust preferred securities
Schedule of Available-for-sale 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	45,002	45,047
More than Twelve Months, Unrealized Losses	(13,434)	(13,384)
Total, Fair Value	45,002	45,047
Total, Unrealized Losses	$ (13,434)	(13,384)
Corporate and other debt securities
Schedule of Available-for-sale 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)