Investment Securities - Amortized Cost, Held to Maturity (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Schedule of Held-to-maturity Securities
Amortized Cost	$ 1,823,622	$ 1,925,572
Gross Unrealized Gains	34,057	31,592
Gross Unrealized Losses	(26,534)	(32,567)
Fair Value	1,831,145	1,924,597
U.S. Treasury securities
Schedule of Held-to-maturity Securities
Amortized Cost	138,714	138,830
Gross Unrealized Gains	8,412	8,665
Gross Unrealized Losses	0	0
Fair Value	147,126	147,495
U.S. government agency securities
Schedule of Held-to-maturity Securities
Amortized Cost	9,984	11,329
Gross Unrealized Gains	204	135
Gross Unrealized Losses	0	0
Fair Value	10,188	11,464
Obligations of states and state agencies
Schedule of Held-to-maturity Securities
Amortized Cost	247,277	252,185
Gross Unrealized Gains	9,412	6,692
Gross Unrealized Losses	(1,449)	(1,428)
Fair Value	255,240	257,449
Municipal bonds
Schedule of Held-to-maturity Securities
Amortized Cost	247,880	314,405
Gross Unrealized Gains	8,049	6,438
Gross Unrealized Losses	(10)	(466)
Fair Value	255,919	320,377
Obligations of states and political subdivisions
Schedule of Held-to-maturity Securities
Amortized Cost	495,157	566,590
Gross Unrealized Gains	17,461	13,130
Gross Unrealized Losses	(1,459)	(1,894)
Fair Value	511,159	577,826
Residential mortgage-backed securities
Schedule of Held-to-maturity Securities
Amortized Cost	1,088,389	1,112,460
Gross Unrealized Gains	7,234	8,432
Gross Unrealized Losses	(13,920)	(18,090)
Fair Value	1,081,703	1,102,802
Trust preferred securities
Schedule of Held-to-maturity Securities
Amortized Cost	49,819	59,804
Gross Unrealized Gains	47	40
Gross Unrealized Losses	(10,868)	(12,554)
Fair Value	38,998	47,290
Corporate and other debt securities
Schedule of Held-to-maturity Securities
Amortized Cost	41,559	36,559
Gross Unrealized Gains	699	1,190
Gross Unrealized Losses	(287)	(29)
Fair Value	$ 41,971	$ 37,720