Investment Securities - Amortized Cost, Gross Unrealized Gains and Losses and Fair Value of Securities Held to Maturity (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Schedule of Held-to-maturity Securities
Held to Maturity, Amortized Cost	$ 1,845,151	$ 1,596,385
Gross Unrealized Gains	62,353	46,217
Gross Unrealized Losses	(15,522)	(21,563)
Held to maturity, fair value	1,891,982	1,621,039
U.S. Treasury securities
Schedule of Held-to-maturity Securities
Held to Maturity, Amortized Cost	138,866	138,978
Gross Unrealized Gains	15,176	10,505
Gross Unrealized Losses	0	0
Held to maturity, fair value	154,042	149,483
U.S. government agency securities
Schedule of Held-to-maturity Securities
Held to Maturity, Amortized Cost	11,549	12,859
Gross Unrealized Gains	557	271
Gross Unrealized Losses	0	0
Held to maturity, fair value	12,106	13,130
Obligations of states and state agencies
Schedule of Held-to-maturity Securities
Held to Maturity, Amortized Cost	192,735	194,547
Gross Unrealized Gains	13,353	10,538
Gross Unrealized Losses	0	(10)
Held to maturity, fair value	206,088	205,075
Municipal bonds
Schedule of Held-to-maturity Securities
Held to Maturity, Amortized Cost	375,477	310,318
Gross Unrealized Gains	13,960	11,955
Gross Unrealized Losses	0	(85)
Held to maturity, fair value	389,437	322,188
Obligations of states and political subdivisions
Schedule of Held-to-maturity Securities
Held to Maturity, Amortized Cost	568,212	504,865
Gross Unrealized Gains	27,313	22,493
Gross Unrealized Losses	0	(95)
Held to maturity, fair value	595,525	527,263
Residential mortgage-backed securities
Schedule of Held-to-maturity Securities
Held to Maturity, Amortized Cost	1,030,165	852,289
Gross Unrealized Gains	17,642	11,018
Gross Unrealized Losses	(1,420)	(8,035)
Held to maturity, fair value	1,046,387	855,272
Trust preferred securities
Schedule of Held-to-maturity Securities
Held to Maturity, Amortized Cost	59,800	59,785
Gross Unrealized Gains	28	36
Gross Unrealized Losses	(14,102)	(13,384)
Held to maturity, fair value	45,726	46,437
Corporate and other debt securities
Schedule of Held-to-maturity Securities
Held to Maturity, Amortized Cost	36,559	27,609
Gross Unrealized Gains	1,637	1,894
Gross Unrealized Losses	0	(49)
Held to maturity, fair value	$ 38,196	$ 29,454