Investment Securities - Age of Unrealized Losses and Fair Value of Related Securities Held to Maturity (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Schedule of Held-to-Maturity Securities [Line Items]
Less than Twelve Months, Fair Value	$ 820,561	$ 263,042
Less than Twelve Months, Unrealized Losses	(16,343)	(3,042)
More than Twelve Months, Fair Value	160,403	219,966
More than Twelve Months, Unrealized Losses	(16,224)	(18,521)
Total, Fair Value	980,964	483,008
Total, Unrealized Losses	(32,567)	(21,563)
Obligations of states and state agencies
Schedule of Held-to-Maturity Securities [Line Items]
Less than Twelve Months, Fair Value	98,114	6,837
Less than Twelve Months, Unrealized Losses	(1,428)	(5)
More than Twelve Months, Fair Value	0	1,965
More than Twelve Months, Unrealized Losses	0	(5)
Total, Fair Value	98,114	8,802
Total, Unrealized Losses	(1,428)	(10)
Municipal bonds
Schedule of Held-to-Maturity Securities [Line Items]
Less than Twelve Months, Fair Value	27,368	8,814
Less than Twelve Months, Unrealized Losses	(466)	(72)
More than Twelve Months, Fair Value	0	10,198
More than Twelve Months, Unrealized Losses	0	(13)
Total, Fair Value	27,368	19,012
Total, Unrealized Losses	(466)	(85)
Total obligations of states and political subdivisions
Schedule of Held-to-Maturity Securities [Line Items]
Less than Twelve Months, Fair Value	125,482	15,651
Less than Twelve Months, Unrealized Losses	(1,894)	(77)
More than Twelve Months, Fair Value	0	12,163
More than Twelve Months, Unrealized Losses	0	(18)
Total, Fair Value	125,482	27,814
Total, Unrealized Losses	(1,894)	(95)
Residential mortgage-backed securities
Schedule of Held-to-Maturity Securities [Line Items]
Less than Twelve Months, Fair Value	692,108	244,440
Less than Twelve Months, Unrealized Losses	(14,420)	(2,916)
More than Twelve Months, Fair Value	114,505	162,756
More than Twelve Months, Unrealized Losses	(3,670)	(5,119)
Total, Fair Value	806,613	407,196
Total, Unrealized Losses	(18,090)	(8,035)
Trust preferred securities
Schedule of Held-to-Maturity Securities [Line Items]
Less than Twelve Months, Fair Value		0
Less than Twelve Months, Unrealized Losses		0
More than Twelve Months, Fair Value	45,898	45,047
More than Twelve Months, Unrealized Losses	(12,554)	(13,384)
Total, Fair Value	45,898	45,047
Total, Unrealized Losses	(12,554)	(13,384)
Corporate and other debt securities
Schedule of Held-to-Maturity Securities [Line Items]
Less than Twelve Months, Fair Value	2,971	2,951
Less than Twelve Months, Unrealized Losses	(29)	(49)
More than Twelve Months, Fair Value	0	0
More than Twelve Months, Unrealized Losses	0	0
Total, Fair Value	2,971	2,951
Total, Unrealized Losses	$ (29)	$ (49)