Investment Securities - Amortized Cost, Gross Unrealized Gains and Losses and Fair Value of Securities Held to Maturity (Detail) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Schedule of Held-to-Maturity Securities [Line Items]
Held to Maturity, Amortized Cost	$ 1,765,220	$ 1,596,385
Gross Unrealized Gains	56,703	46,217
Gross Unrealized Losses	(18,212)	(21,563)
Held to maturity, fair value	1,803,711	1,621,039
U.S. Treasury securities
Schedule of Held-to-Maturity Securities [Line Items]
Held to Maturity, Amortized Cost	138,904	138,978
Gross Unrealized Gains	13,868	10,505
Gross Unrealized Losses	0	0
Held to maturity, fair value	152,772	149,483
U.S. government agency securities
Schedule of Held-to-Maturity Securities [Line Items]
Held to Maturity, Amortized Cost	12,076	12,859
Gross Unrealized Gains	487	271
Gross Unrealized Losses	0	0
Held to maturity, fair value	12,563	13,130
Obligations of states and state agencies
Schedule of Held-to-Maturity Securities [Line Items]
Held to Maturity, Amortized Cost	193,343	194,547
Gross Unrealized Gains	13,148	10,538
Gross Unrealized Losses	0	(10)
Held to maturity, fair value	206,491	205,075
Municipal bonds
Schedule of Held-to-Maturity Securities [Line Items]
Held to Maturity, Amortized Cost	372,166	310,318
Gross Unrealized Gains	13,268	11,955
Gross Unrealized Losses	0	(85)
Held to maturity, fair value	385,434	322,188
Obligations of states and political subdivisions
Schedule of Held-to-Maturity Securities [Line Items]
Held to Maturity, Amortized Cost	565,509	504,865
Gross Unrealized Gains	26,416	22,493
Gross Unrealized Losses	0	(95)
Held to maturity, fair value	591,925	527,263
Residential mortgage-backed securities
Schedule of Held-to-Maturity Securities [Line Items]
Held to Maturity, Amortized Cost	957,377	852,289
Gross Unrealized Gains	14,173	11,018
Gross Unrealized Losses	(4,408)	(8,035)
Held to maturity, fair value	967,142	855,272
Trust preferred securities
Schedule of Held-to-Maturity Securities [Line Items]
Held to Maturity, Amortized Cost	59,795	59,785
Gross Unrealized Gains	39	36
Gross Unrealized Losses	(13,804)	(13,384)
Held to maturity, fair value	46,030	46,437
Corporate and other debt securities
Schedule of Held-to-Maturity Securities [Line Items]
Held to Maturity, Amortized Cost	31,559	27,609
Gross Unrealized Gains	1,720	1,894
Gross Unrealized Losses	0	(49)
Held to maturity, fair value	$ 33,279	$ 29,454