Investment Securities - Age of Unrealized Losses and Fair Value of Related Securities Held to Maturity (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Schedule of Held-to-Maturity Securities [Line Items]
Less than twelve months, fair value	$ 101,621	$ 365,182
Less than twelve months, unrealized losses	(1,207)	(2,702)
More than twelve months, fair value	1,086,150	663,238
More than twelve months, unrealized losses	(46,532)	(27,788)
Total, fair value	1,187,771	1,028,420
Total, unrealized losses	(47,739)	(30,490)
U.S. government agency 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	6,074
More than twelve months, unrealized losses	(135)
Total, fair value	6,074
Total, unrealized losses	(135)
Obligations of states and state agencies
Schedule of Held-to-Maturity Securities [Line Items]
Less than twelve months, fair value	16,098	6,342
Less than twelve months, unrealized losses	(266)	(50)
More than twelve months, fair value	138,437	53,034
More than twelve months, unrealized losses	(5,469)	(1,603)
Total, fair value	154,535	59,376
Total, unrealized losses	(5,735)	(1,653)
Municipal bonds
Schedule of Held-to-Maturity Securities [Line Items]
Less than twelve months, fair value	3,335	4,644
Less than twelve months, unrealized losses	(37)	(25)
More than twelve months, fair value	60,078	561
More than twelve months, unrealized losses	(1,324)	(3)
Total, fair value	63,413	5,205
Total, unrealized losses	(1,361)	(28)
Total obligations of states and political subdivisions
Schedule of Held-to-Maturity Securities [Line Items]
Less than twelve months, fair value	19,433	10,986
Less than twelve months, unrealized losses	(303)	(75)
More than twelve months, fair value	198,515	53,595
More than twelve months, unrealized losses	(6,793)	(1,606)
Total, fair value	217,948	64,581
Total, unrealized losses	(7,096)	(1,681)
Residential mortgage-backed securities
Schedule of Held-to-Maturity Securities [Line Items]
Less than twelve months, fair value	72,240	344,216
Less than twelve months, unrealized losses	(852)	(2,357)
More than twelve months, fair value	846,671	570,969
More than twelve months, unrealized losses	(33,516)	(16,386)
Total, fair value	918,911	915,185
Total, unrealized losses	(34,368)	(18,743)
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	30,055	38,674
More than twelve months, unrealized losses	(5,923)	(9,796)
Total, fair value	30,055	38,674
Total, unrealized losses	(5,923)	(9,796)
Corporate and other debt securities
Schedule of Held-to-Maturity Securities [Line Items]
Less than twelve months, fair value	9,948	9,980
Less than twelve months, unrealized losses	(52)	(270)
More than twelve months, fair value	4,835	0
More than twelve months, unrealized losses	(165)	0
Total, fair value	14,783	9,980
Total, unrealized losses	$ (217)	$ (270)