Investment Securities - Amortized Cost, Gross Unrealized Gains and Losses and Fair Value of Securities Held to Maturity (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Schedule of Held-to-Maturity Securities [Line Items]
Held to Maturity, Amortized Cost	$ 1,826,143	$ 1,731,737
Gross Unrealized Gains	54,444	30,962
Gross Unrealized Losses	(26,466)	(51,272)
Fair Value	1,854,121	1,711,427
U.S. Treasury Securities [Member]
Schedule of Held-to-Maturity Securities [Line Items]
Held to Maturity, Amortized Cost	139,192	139,255
Gross Unrealized Gains	9,936	5,567
Gross Unrealized Losses	0	(515)
Fair Value	149,128	144,307
U.S. Government Agency Securities [Member]
Schedule of Held-to-Maturity Securities [Line Items]
Held to Maturity, Amortized Cost	14,346	4,427
Gross Unrealized Gains	372	0
Gross Unrealized Losses	0	(62)
Fair Value	14,718	4,365
Obligations of States and State Agencies [Member]
Schedule of Held-to-Maturity Securities [Line Items]
Held to Maturity, Amortized Cost	198,610	192,653
Gross Unrealized Gains	7,056	1,944
Gross Unrealized Losses	(1,291)	(5,473)
Fair Value	204,375	189,124
Municipal Bonds [Member]
Schedule of Held-to-Maturity Securities [Line Items]
Held to Maturity, Amortized Cost	316,792	353,233
Gross Unrealized Gains	10,441	6,053
Gross Unrealized Losses	(1,215)	(5,259)
Fair Value	326,018	354,027
Obligations of States and Political Subdivisions [Member]
Schedule of Held-to-Maturity Securities [Line Items]
Held to Maturity, Amortized Cost	515,402	545,886
Gross Unrealized Gains	17,497	7,997
Gross Unrealized Losses	(2,506)	(10,732)
Fair Value	530,393	543,151
Residential Mortgage-Backed Securities [Member]
Schedule of Held-to-Maturity Securities [Line Items]
Held to Maturity, Amortized Cost	1,019,086	886,043
Gross Unrealized Gains	20,233	12,609
Gross Unrealized Losses	(11,199)	(27,631)
Fair Value	1,028,120	871,021
Trust Preferred Securities [Member]
Schedule of Held-to-Maturity Securities [Line Items]
Held to Maturity, Amortized Cost	98,452	103,458
Gross Unrealized Gains	193	363
Gross Unrealized Losses	(12,759)	(12,332)
Fair Value	85,886	91,489
Corporate and Other Debt Securities [Member]
Schedule of Held-to-Maturity Securities [Line Items]
Held to Maturity, Amortized Cost	39,665	52,668
Gross Unrealized Gains	6,213	4,426
Gross Unrealized Losses	(2)	0
Fair Value	$ 45,876	$ 57,094