Investment Securities - Unrealized Losses and Fair Value of Held to Maturity (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Schedule of Held-to-maturity Securities
Less than Twelve Months, Fair Value	$ 616,446	$ 820,561
Less than Twelve Months, Unrealized Losses	(9,657)	(16,343)
More than Twelve Months, Fair Value	265,994	160,403
More than Twelve Months, Unrealized Losses	(16,877)	(16,224)
Total, Fair Value	882,440	980,964
Total, Unrealized Losses	(26,534)	(32,567)
Obligations of states and state agencies
Schedule of Held-to-maturity Securities
Less than Twelve Months, Fair Value	55,565	98,114
Less than Twelve Months, Unrealized Losses	(1,449)	(1,428)
More than Twelve Months, Fair Value	0	0
More than Twelve Months, Unrealized Losses	0	0
Total, Fair Value	55,565	98,114
Total, Unrealized Losses	(1,449)	(1,428)
Municipal bonds
Schedule of Held-to-maturity Securities
Less than Twelve Months, Fair Value	4,678	27,368
Less than Twelve Months, Unrealized Losses	(10)	(466)
More than Twelve Months, Fair Value	0	0
More than Twelve Months, Unrealized Losses	0	0
Total, Fair Value	4,678	27,368
Total, Unrealized Losses	(10)	(466)
Obligations of states and political subdivisions
Schedule of Held-to-maturity Securities
Less than Twelve Months, Fair Value	60,243	125,482
Less than Twelve Months, Unrealized Losses	(1,459)	(1,894)
More than Twelve Months, Fair Value	0	0
More than Twelve Months, Unrealized Losses	0	0
Total, Fair Value	60,243	125,482
Total, Unrealized Losses	(1,459)	(1,894)
Residential mortgage-backed securities
Schedule of Held-to-maturity Securities
Less than Twelve Months, Fair Value	551,490	692,108
Less than Twelve Months, Unrealized Losses	(7,911)	(14,420)
More than Twelve Months, Fair Value	228,397	114,505
More than Twelve Months, Unrealized Losses	(6,009)	(3,670)
Total, Fair Value	779,887	806,613
Total, Unrealized Losses	(13,920)	(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	37,597	45,898
More than Twelve Months, Unrealized Losses	(10,868)	(12,554)
Total, Fair Value	37,597	45,898
Total, Unrealized Losses	(10,868)	(12,554)
Corporate and other debt securities
Schedule of Held-to-maturity Securities
Less than Twelve Months, Fair Value	4,713	2,971
Less than Twelve Months, Unrealized Losses	(287)	(29)
More than Twelve Months, Fair Value	0	0
More than Twelve Months, Unrealized Losses	0	0
Total, Fair Value	4,713	2,971
Total, Unrealized Losses	$ (287)	$ (29)