Investment Securities - Unrealized Losses and Fair Value of Held to Maturity (Detail) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Schedule of Held-to-maturity Securities
Less than Twelve Months, Fair Value	$ 695,410	$ 820,561
Less than Twelve Months, Unrealized Losses	(13,892)	(16,343)
More than Twelve Months, Fair Value	193,857	160,403
More than Twelve Months, Unrealized Losses	(15,505)	(16,224)
Total, Fair Value	889,267	980,964
Total, Unrealized Losses	(29,397)	(32,567)
Obligations of states and state agencies
Schedule of Held-to-maturity Securities
Less than Twelve Months, Fair Value	63,537	98,114
Less than Twelve Months, Unrealized Losses	(1,688)	(1,428)
More than Twelve Months, Fair Value	0	0
More than Twelve Months, Unrealized Losses	0	0
Total, Fair Value	63,537	98,114
Total, Unrealized Losses	(1,688)	(1,428)
Municipal bonds
Schedule of Held-to-maturity Securities
Less than Twelve Months, Fair Value	4,664	27,368
Less than Twelve Months, Unrealized Losses	(26)	(466)
More than Twelve Months, Fair Value	0	0
More than Twelve Months, Unrealized Losses	0	0
Total, Fair Value	4,664	27,368
Total, Unrealized Losses	(26)	(466)
Obligations of states and political subdivisions
Schedule of Held-to-maturity Securities
Less than Twelve Months, Fair Value	68,201	125,482
Less than Twelve Months, Unrealized Losses	(1,714)	(1,894)
More than Twelve Months, Fair Value	0	0
More than Twelve Months, Unrealized Losses	0	0
Total, Fair Value	68,201	125,482
Total, Unrealized Losses	(1,714)	(1,894)
Residential mortgage-backed securities
Schedule of Held-to-maturity Securities
Less than Twelve Months, Fair Value	622,527	692,108
Less than Twelve Months, Unrealized Losses	(11,860)	(14,420)
More than Twelve Months, Fair Value	156,974	114,505
More than Twelve Months, Unrealized Losses	(3,927)	(3,670)
Total, Fair Value	779,501	806,613
Total, Unrealized Losses	(15,787)	(18,090)
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	36,883	45,898
More than Twelve Months, Unrealized Losses	(11,578)	(12,554)
Total, Fair Value	36,883	45,898
Total, Unrealized Losses	(11,578)	(12,554)
Corporate and other debt securities
Schedule of Held-to-maturity Securities
Less than Twelve Months, Fair Value	4,682	2,971
Less than Twelve Months, Unrealized Losses	(318)	(29)
More than Twelve Months, Fair Value	0	0
More than Twelve Months, Unrealized Losses	0	0
Total, Fair Value	4,682	2,971
Total, Unrealized Losses	$ (318)	$ (29)