Investment Securities - Age of Unrealized Losses and Fair Value of Related Securities Held to Maturity (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule of Held-to-Maturity Securities [Line Items]
Less than twelve months, fair value	$ 365,182	$ 820,561
Less than twelve months, unrealized losses	(2,702)	(16,343)
More than twelve months, fair value	663,238	160,403
More than twelve months, unrealized losses	(27,788)	(16,224)
Total, fair value	1,028,420	980,964
Total, unrealized losses	(30,490)	(32,567)
Obligations of states and state agencies
Schedule of Held-to-Maturity Securities [Line Items]
Less than twelve months, fair value	6,342	98,114
Less than twelve months, unrealized losses	(50)	(1,428)
More than twelve months, fair value	53,034	0
More than twelve months, unrealized losses	(1,603)	0
Total, fair value	59,376	98,114
Total, unrealized losses	(1,653)	(1,428)
Municipal bonds
Schedule of Held-to-Maturity Securities [Line Items]
Less than twelve months, fair value	4,644	27,368
Less than twelve months, unrealized losses	(25)	(466)
More than twelve months, fair value	561	0
More than twelve months, unrealized losses	(3)	0
Total, fair value	5,205	27,368
Total, unrealized losses	(28)	(466)
Total obligations of states and political subdivisions
Schedule of Held-to-Maturity Securities [Line Items]
Less than twelve months, fair value	10,986	125,482
Less than twelve months, unrealized losses	(75)	(1,894)
More than twelve months, fair value	53,595	0
More than twelve months, unrealized losses	(1,606)	0
Total, fair value	64,581	125,482
Total, unrealized losses	(1,681)	(1,894)
Residential mortgage-backed securities
Schedule of Held-to-Maturity Securities [Line Items]
Less than twelve months, fair value	344,216	692,108
Less than twelve months, unrealized losses	(2,357)	(14,420)
More than twelve months, fair value	570,969	114,505
More than twelve months, unrealized losses	(16,386)	(3,670)
Total, fair value	915,185	806,613
Total, unrealized losses	(18,743)	(18,090)
Trust preferred securities
Schedule of Held-to-Maturity Securities [Line Items]
Less than twelve months, fair value	0	0
Less than twelve months, unrealized losses	0	0
More than twelve months, fair value	38,674	45,898
More than twelve months, unrealized losses	(9,796)	(12,554)
Total, fair value	38,674	45,898
Total, unrealized losses	(9,796)	(12,554)
Corporate and other debt securities
Schedule of Held-to-Maturity Securities [Line Items]
Less than twelve months, fair value	9,980	2,971
Less than twelve months, unrealized losses	(270)	(29)
More than twelve months, fair value	0	0
More than twelve months, unrealized losses	0	0
Total, fair value	9,980	2,971
Total, unrealized losses	$ (270)	$ (29)