Investment Securities - Unrealized Losses and Fair Value of Held to Maturity (Detail) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Schedule of Held-to-maturity Securities
Less than Twelve Months, Fair Value	$ 745,354	$ 820,561
Less than Twelve Months, Unrealized Losses	(15,763)	(16,343)
More than Twelve Months, Fair Value	169,413	160,403
More than Twelve Months, Unrealized Losses	(15,230)	(16,224)
Total, Fair Value	914,767	980,964
Total, Unrealized Losses	(30,993)	(32,567)
Obligations of states and state agencies
Schedule of Held-to-maturity Securities
Less than Twelve Months, Fair Value	75,169	98,114
Less than Twelve Months, Unrealized Losses	(1,362)	(1,428)
More than Twelve Months, Fair Value	0	0
More than Twelve Months, Unrealized Losses	0	0
Total, Fair Value	75,169	98,114
Total, Unrealized Losses	(1,362)	(1,428)
Municipal bonds
Schedule of Held-to-maturity Securities
Less than Twelve Months, Fair Value	21,820	27,368
Less than Twelve Months, Unrealized Losses	(222)	(466)
More than Twelve Months, Fair Value	0	0
More than Twelve Months, Unrealized Losses	0	0
Total, Fair Value	21,820	27,368
Total, Unrealized Losses	(222)	(466)
Obligations of states and political subdivisions
Schedule of Held-to-maturity Securities
Less than Twelve Months, Fair Value	96,989	125,482
Less than Twelve Months, Unrealized Losses	(1,584)	(1,894)
More than Twelve Months, Fair Value	0	0
More than Twelve Months, Unrealized Losses	0	0
Total, Fair Value	96,989	125,482
Total, Unrealized Losses	(1,584)	(1,894)
Residential mortgage-backed securities
Schedule of Held-to-maturity Securities
Less than Twelve Months, Fair Value	648,365	692,108
Less than Twelve Months, Unrealized Losses	(14,179)	(14,420)
More than Twelve Months, Fair Value	122,764	114,505
More than Twelve Months, Unrealized Losses	(3,422)	(3,670)
Total, Fair Value	771,129	806,613
Total, Unrealized Losses	(17,601)	(18,090)
Trust preferred securities
Schedule of Held-to-maturity Securities
Less than Twelve Months, Fair Value	0
Less than Twelve Months, Unrealized Losses	0
More than Twelve Months, Fair Value	46,649	45,898
More than Twelve Months, Unrealized Losses	(11,808)	(12,554)
Total, Fair Value	46,649	45,898
Total, Unrealized Losses	$ (11,808)	(12,554)
Corporate and other debt securities
Schedule of Held-to-maturity Securities
Less than Twelve Months, Fair Value		2,971
Less than Twelve Months, Unrealized Losses		(29)
More than Twelve Months, Fair Value		0
More than Twelve Months, Unrealized Losses		0
Total, Fair Value		2,971
Total, Unrealized Losses		$ (29)