Investment Securities - Amortized Cost, Held to Maturity (Detail) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Schedule of Held-to-maturity Securities
Amortized Cost	$ 1,902,329	$ 1,925,572
Gross Unrealized Gains	33,187	31,592
Gross Unrealized Losses	(30,993)	(32,567)
Fair Value	1,904,523	1,924,597
U.S. Treasury securities
Schedule of Held-to-maturity Securities
Amortized Cost	138,793	138,830
Gross Unrealized Gains	8,626	8,665
Gross Unrealized Losses	0	0
Fair Value	147,419	147,495
U.S. government agency securities
Schedule of Held-to-maturity Securities
Amortized Cost	10,722	11,329
Gross Unrealized Gains	159	135
Gross Unrealized Losses	0	0
Fair Value	10,881	11,464
Obligations of states and state agencies
Schedule of Held-to-maturity Securities
Amortized Cost	250,905	252,185
Gross Unrealized Gains	8,005	6,692
Gross Unrealized Losses	(1,362)	(1,428)
Fair Value	257,548	257,449
Municipal bonds
Schedule of Held-to-maturity Securities
Amortized Cost	287,127	314,405
Gross Unrealized Gains	7,335	6,438
Gross Unrealized Losses	(222)	(466)
Fair Value	294,240	320,377
Obligations of states and political subdivisions
Schedule of Held-to-maturity Securities
Amortized Cost	538,032	566,590
Gross Unrealized Gains	15,340	13,130
Gross Unrealized Losses	(1,584)	(1,894)
Fair Value	551,788	577,826
Residential mortgage-backed securities
Schedule of Held-to-maturity Securities
Amortized Cost	1,113,413	1,112,460
Gross Unrealized Gains	7,958	8,432
Gross Unrealized Losses	(17,601)	(18,090)
Fair Value	1,103,770	1,102,802
Trust preferred securities
Schedule of Held-to-maturity Securities
Amortized Cost	59,810	59,804
Gross Unrealized Gains	32	40
Gross Unrealized Losses	(11,808)	(12,554)
Fair Value	48,034	47,290
Corporate and other debt securities
Schedule of Held-to-maturity Securities
Amortized Cost	41,559	36,559
Gross Unrealized Gains	1,072	1,190
Gross Unrealized Losses	0	(29)
Fair Value	$ 42,631	$ 37,720