Investment Securities - Amortized Cost, Held to Maturity (Detail) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Schedule of Held-to-maturity Securities
Amortized Cost	$ 1,822,263	$ 1,925,572
Gross Unrealized Gains	35,866	31,592
Gross Unrealized Losses	(29,397)	(32,567)
Fair Value	1,828,732	1,924,597
U.S. Treasury securities
Schedule of Held-to-maturity Securities
Amortized Cost	138,754	138,830
Gross Unrealized Gains	8,902	8,665
Gross Unrealized Losses	0	0
Fair Value	147,656	147,495
U.S. government agency securities
Schedule of Held-to-maturity Securities
Amortized Cost	10,597	11,329
Gross Unrealized Gains	205	135
Gross Unrealized Losses	0	0
Fair Value	10,802	11,464
Obligations of states and state agencies
Schedule of Held-to-maturity Securities
Amortized Cost	249,607	252,185
Gross Unrealized Gains	9,623	6,692
Gross Unrealized Losses	(1,688)	(1,428)
Fair Value	257,542	257,449
Municipal bonds
Schedule of Held-to-maturity Securities
Amortized Cost	251,795	314,405
Gross Unrealized Gains	8,519	6,438
Gross Unrealized Losses	(26)	(466)
Fair Value	260,288	320,377
Obligations of states and political subdivisions
Schedule of Held-to-maturity Securities
Amortized Cost	501,402	566,590
Gross Unrealized Gains	18,142	13,130
Gross Unrealized Losses	(1,714)	(1,894)
Fair Value	517,830	577,826
Residential mortgage-backed securities
Schedule of Held-to-maturity Securities
Amortized Cost	1,070,137	1,112,460
Gross Unrealized Gains	7,658	8,432
Gross Unrealized Losses	(15,787)	(18,090)
Fair Value	1,062,008	1,102,802
Trust preferred securities
Schedule of Held-to-maturity Securities
Amortized Cost	59,814	59,804
Gross Unrealized Gains	32	40
Gross Unrealized Losses	(11,578)	(12,554)
Fair Value	48,268	47,290
Corporate and other debt securities
Schedule of Held-to-maturity Securities
Amortized Cost	41,559	36,559
Gross Unrealized Gains	927	1,190
Gross Unrealized Losses	(318)	(29)
Fair Value	$ 42,168	$ 37,720