Investment Securities - Amortized Cost, Gross Unrealized Gains and Losses and Fair Value of Securities Held to Maturity (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Schedule of Held-to-Maturity Securities [Line Items]
Held to Maturity, Amortized Cost	$ 1,813,096	$ 1,731,737
Gross Unrealized Gains	40,803	30,962
Gross Unrealized Losses	(37,850)	(51,272)
Fair Value	1,816,049	1,711,427
U.S. Treasury Securities [Member]
Schedule of Held-to-Maturity Securities [Line Items]
Held to Maturity, Amortized Cost	139,227	139,255
Gross Unrealized Gains	7,669	5,567
Gross Unrealized Losses	0	(515)
Fair Value	146,896	144,307
U.S. Government Agency Securities [Member]
Schedule of Held-to-Maturity Securities [Line Items]
Held to Maturity, Amortized Cost	14,454	4,427
Gross Unrealized Gains	47	0
Gross Unrealized Losses	(13)	(62)
Fair Value	14,488	4,365
Obligations of States and State Agencies [Member]
Schedule of Held-to-Maturity Securities [Line Items]
Held to Maturity, Amortized Cost	199,190	192,653
Gross Unrealized Gains	4,912	1,944
Gross Unrealized Losses	(2,694)	(5,473)
Fair Value	201,408	189,124
Municipal Bonds [Member]
Schedule of Held-to-Maturity Securities [Line Items]
Held to Maturity, Amortized Cost	332,741	353,233
Gross Unrealized Gains	8,204	6,053
Gross Unrealized Losses	(2,695)	(5,259)
Fair Value	338,250	354,027
Obligations of States and Political Subdivisions [Member]
Schedule of Held-to-Maturity Securities [Line Items]
Held to Maturity, Amortized Cost	531,931	545,886
Gross Unrealized Gains	13,116	7,997
Gross Unrealized Losses	(5,389)	(10,732)
Fair Value	539,658	543,151
Residential Mortgage-Backed Securities [Member]
Schedule of Held-to-Maturity Securities [Line Items]
Held to Maturity, Amortized Cost	971,361	886,043
Gross Unrealized Gains	14,908	12,609
Gross Unrealized Losses	(20,219)	(27,631)
Fair Value	966,050	871,021
Trust Preferred Securities [Member]
Schedule of Held-to-Maturity Securities [Line Items]
Held to Maturity, Amortized Cost	98,450	103,458
Gross Unrealized Gains	189	363
Gross Unrealized Losses	(12,204)	(12,332)
Fair Value	86,435	91,489
Corporate and Other Debt Securities [Member]
Schedule of Held-to-Maturity Securities [Line Items]
Held to Maturity, Amortized Cost	57,673	52,668
Gross Unrealized Gains	4,874	4,426
Gross Unrealized Losses	(25)	0
Fair Value	$ 62,522	$ 57,094