Investment Securities - Amortized Cost, Gross Unrealized Gains and Losses and Fair Value of Securities Held to Maturity (Detail) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Schedule of Held-to-Maturity Securities [Line Items]
Held to Maturity, Amortized Cost	$ 1,618,466	$ 1,596,385
Gross Unrealized Gains	58,854	46,217
Gross Unrealized Losses	(17,096)	(21,563)
Held to maturity, fair value	1,660,224	1,621,039
U.S. Treasury securities
Schedule of Held-to-Maturity Securities [Line Items]
Held to Maturity, Amortized Cost	138,942	138,978
Gross Unrealized Gains	14,935	10,505
Gross Unrealized Losses	0	0
Held to maturity, fair value	153,877	149,483
U.S. government agency securities
Schedule of Held-to-Maturity Securities [Line Items]
Held to Maturity, Amortized Cost	12,225	12,859
Gross Unrealized Gains	513	271
Gross Unrealized Losses	0	0
Held to maturity, fair value	12,738	13,130
Obligations of states and state agencies
Schedule of Held-to-Maturity Securities [Line Items]
Held to Maturity, Amortized Cost	193,947	194,547
Gross Unrealized Gains	12,776	10,538
Gross Unrealized Losses	0	(10)
Held to maturity, fair value	206,723	205,075
Municipal bonds
Schedule of Held-to-Maturity Securities [Line Items]
Held to Maturity, Amortized Cost	322,698	310,318
Gross Unrealized Gains	13,648	11,955
Gross Unrealized Losses	0	(85)
Held to maturity, fair value	336,346	322,188
Obligations of states and political subdivisions
Schedule of Held-to-Maturity Securities [Line Items]
Held to Maturity, Amortized Cost	516,645	504,865
Gross Unrealized Gains	26,424	22,493
Gross Unrealized Losses	0	(95)
Held to maturity, fair value	543,069	527,263
Residential mortgage-backed securities
Schedule of Held-to-Maturity Securities [Line Items]
Held to Maturity, Amortized Cost	859,305	852,289
Gross Unrealized Gains	15,068	11,018
Gross Unrealized Losses	(3,662)	(8,035)
Held to maturity, fair value	870,711	855,272
Trust preferred securities
Schedule of Held-to-Maturity Securities [Line Items]
Held to Maturity, Amortized Cost	59,790	59,785
Gross Unrealized Gains	48	36
Gross Unrealized Losses	(13,434)	(13,384)
Held to maturity, fair value	46,404	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	1,866	1,894
Gross Unrealized Losses	0	(49)
Held to maturity, fair value	$ 33,425	$ 29,454