Investment Securities - Amortized Cost, Gross Unrealized Gains and Losses and Fair Value of Securities Held to Maturity (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Schedule of Held-to-Maturity Securities [Line Items]
Held to Maturity, Amortized Cost	$ 1,813,312	$ 1,731,737
Gross Unrealized Gains	50,942	30,962
Gross Unrealized Losses	(25,622)	(51,272)
Fair Value	1,838,632	1,711,427
U.S. Treasury Securities [Member]
Schedule of Held-to-Maturity Securities [Line Items]
Held to Maturity, Amortized Cost	139,156	139,255
Gross Unrealized Gains	9,329	5,567
Gross Unrealized Losses	0	(515)
Fair Value	148,485	144,307
U.S. Government Agency Securities [Member]
Schedule of Held-to-Maturity Securities [Line Items]
Held to Maturity, Amortized Cost	13,881	4,427
Gross Unrealized Gains	326	0
Gross Unrealized Losses	0	(62)
Fair Value	14,207	4,365
Obligations of States and State Agencies [Member]
Schedule of Held-to-Maturity Securities [Line Items]
Held to Maturity, Amortized Cost	198,027	192,653
Gross Unrealized Gains	8,494	1,944
Gross Unrealized Losses	(419)	(5,473)
Fair Value	206,102	189,124
Municipal Bonds [Member]
Schedule of Held-to-Maturity Securities [Line Items]
Held to Maturity, Amortized Cost	303,285	353,233
Gross Unrealized Gains	10,965	6,053
Gross Unrealized Losses	(597)	(5,259)
Fair Value	313,653	354,027
Obligations of States and Political Subdivisions [Member]
Schedule of Held-to-Maturity Securities [Line Items]
Held to Maturity, Amortized Cost	501,312	545,886
Gross Unrealized Gains	19,459	7,997
Gross Unrealized Losses	(1,016)	(10,732)
Fair Value	519,755	543,151
Residential Mortgage-Backed Securities [Member]
Schedule of Held-to-Maturity Securities [Line Items]
Held to Maturity, Amortized Cost	1,020,852	886,043
Gross Unrealized Gains	15,988	12,609
Gross Unrealized Losses	(12,438)	(27,631)
Fair Value	1,024,402	871,021
Trust Preferred Securities [Member]
Schedule of Held-to-Maturity Securities [Line Items]
Held to Maturity, Amortized Cost	98,454	103,458
Gross Unrealized Gains	185	363
Gross Unrealized Losses	(12,165)	(12,332)
Fair Value	86,474	91,489
Corporate and Other Debt Securities [Member]
Schedule of Held-to-Maturity Securities [Line Items]
Held to Maturity, Amortized Cost	39,657	52,668
Gross Unrealized Gains	5,655	4,426
Gross Unrealized Losses	(3)	0
Fair Value	$ 45,309	$ 57,094