Investment Securities - Age of Unrealized Losses and Fair Value of Related Securities Held to Maturity (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Schedule of Held-to-maturity Securities
Less than Twelve Months, Fair Value	$ 185,774	$ 263,042
Less than Twelve Months, Unrealized Losses	(867)	(3,042)
More than Twelve Months, Fair Value	172,555	219,966
More than Twelve Months, Unrealized Losses	(14,655)	(18,521)
Total, Fair Value	358,329	483,008
Total, Unrealized Losses	(15,522)	(21,563)
Obligations of states and state agencies
Schedule of Held-to-maturity Securities
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 Held-to-maturity Securities
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 Held-to-maturity Securities
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 Held-to-maturity Securities
Less than Twelve Months, Fair Value	185,774	244,440
Less than Twelve Months, Unrealized Losses	(867)	(2,916)
More than Twelve Months, Fair Value	128,211	162,756
More than Twelve Months, Unrealized Losses	(553)	(5,119)
Total, Fair Value	313,985	407,196
Total, Unrealized Losses	(1,420)	(8,035)
Trust preferred securities
Schedule of Held-to-maturity Securities
Less than Twelve Months, Fair Value	0	0
Less than Twelve Months, Unrealized Losses	0	0
More than Twelve Months, Fair Value	44,344	45,047
More than Twelve Months, Unrealized Losses	(14,102)	(13,384)
Total, Fair Value	44,344	45,047
Total, Unrealized Losses	$ (14,102)	(13,384)
Corporate and other debt securities
Schedule of Held-to-maturity Securities
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)