Investment Securities - Amortized Cost, Gross Unrealized Gains and Losses and Fair Value of Investment Securities Held to Maturity (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Schedule of Held-to-Maturity Securities [Line Items]
Amortized Cost	$ 1,925,572	$ 1,596,385
Gross Unrealized Gains	31,592	46,217
Gross Unrealized Losses	(32,567)	(21,563)
Fair Value	1,924,597	1,621,039
U.S. Treasury securities
Schedule of Held-to-Maturity Securities [Line Items]
Amortized Cost	138,830	138,978
Gross Unrealized Gains	8,665	10,505
Gross Unrealized Losses	0	0
Fair Value	147,495	149,483
U.S. government agency securities
Schedule of Held-to-Maturity Securities [Line Items]
Amortized Cost	11,329	12,859
Gross Unrealized Gains	135	271
Gross Unrealized Losses	0	0
Fair Value	11,464	13,130
Obligations of states and state agencies
Schedule of Held-to-Maturity Securities [Line Items]
Amortized Cost	252,185	194,547
Gross Unrealized Gains	6,692	10,538
Gross Unrealized Losses	(1,428)	(10)
Fair Value	257,449	205,075
Municipal bonds
Schedule of Held-to-Maturity Securities [Line Items]
Amortized Cost	314,405	310,318
Gross Unrealized Gains	6,438	11,955
Gross Unrealized Losses	(466)	(85)
Fair Value	320,377	322,188
Total obligations of states and political subdivisions
Schedule of Held-to-Maturity Securities [Line Items]
Amortized Cost	566,590	504,865
Gross Unrealized Gains	13,130	22,493
Gross Unrealized Losses	(1,894)	(95)
Fair Value	577,826	527,263
Residential mortgage-backed securities
Schedule of Held-to-Maturity Securities [Line Items]
Amortized Cost	1,112,460	852,289
Gross Unrealized Gains	8,432	11,018
Gross Unrealized Losses	(18,090)	(8,035)
Fair Value	1,102,802	855,272
Trust preferred securities
Schedule of Held-to-Maturity Securities [Line Items]
Amortized Cost	59,804	59,785
Gross Unrealized Gains	40	36
Gross Unrealized Losses	(12,554)	(13,384)
Fair Value	47,290	46,437
Corporate and other debt securities
Schedule of Held-to-Maturity Securities [Line Items]
Amortized Cost	36,559	27,609
Gross Unrealized Gains	1,190	1,894
Gross Unrealized Losses	(29)	(49)
Fair Value	$ 37,720	$ 29,454