Investment Securities - Amortized Cost, Gross Unrealized Gains and Losses and Fair Value of Investment Securities Held to Maturity (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule of Held-to-Maturity Securities [Line Items]
Amortized Cost	$ 1,842,691	$ 1,925,572
Gross Unrealized Gains	25,419	31,592
Gross Unrealized Losses	(30,490)	(32,567)
Fair Value	1,837,620	1,924,597
U.S. Treasury securities
Schedule of Held-to-Maturity Securities [Line Items]
Amortized Cost	138,676	138,830
Gross Unrealized Gains	6,581	8,665
Gross Unrealized Losses	0	0
Fair Value	145,257	147,495
U.S. government agency securities
Schedule of Held-to-Maturity Securities [Line Items]
Amortized Cost	9,859	11,329
Gross Unrealized Gains	122	135
Gross Unrealized Losses	0	0
Fair Value	9,981	11,464
Obligations of states and state agencies
Schedule of Held-to-Maturity Securities [Line Items]
Amortized Cost	244,272	252,185
Gross Unrealized Gains	7,083	6,692
Gross Unrealized Losses	(1,653)	(1,428)
Fair Value	249,702	257,449
Municipal bonds
Schedule of Held-to-Maturity Securities [Line Items]
Amortized Cost	221,606	314,405
Gross Unrealized Gains	6,199	6,438
Gross Unrealized Losses	(28)	(466)
Fair Value	227,777	320,377
Total obligations of states and political subdivisions
Schedule of Held-to-Maturity Securities [Line Items]
Amortized Cost	465,878	566,590
Gross Unrealized Gains	13,282	13,130
Gross Unrealized Losses	(1,681)	(1,894)
Fair Value	477,479	577,826
Residential mortgage-backed securities
Schedule of Held-to-Maturity Securities [Line Items]
Amortized Cost	1,131,945	1,112,460
Gross Unrealized Gains	4,842	8,432
Gross Unrealized Losses	(18,743)	(18,090)
Fair Value	1,118,044	1,102,802
Trust preferred securities
Schedule of Held-to-Maturity Securities [Line Items]
Amortized Cost	49,824	59,804
Gross Unrealized Gains	60	40
Gross Unrealized Losses	(9,796)	(12,554)
Fair Value	40,088	47,290
Corporate and other debt securities
Schedule of Held-to-Maturity Securities [Line Items]
Amortized Cost	46,509	36,559
Gross Unrealized Gains	532	1,190
Gross Unrealized Losses	(270)	(29)
Fair Value	$ 46,771	$ 37,720