Investment Securities - Amortized Cost, Gross Unrealized Gains and Losses and Fair Value of Investment Securities Held to Maturity (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Schedule of Held-to-Maturity Securities [Line Items]
Amortized Cost	$ 1,731,737	$ 1,599,707
Gross Unrealized Gains	30,962	73,573
Gross Unrealized Losses	(51,272)	(15,330)
Fair Value	1,711,427	1,657,950
U.S. Treasury securities [Member]
Schedule of Held-to-Maturity Securities [Line Items]
Amortized Cost	139,255	99,869
Gross Unrealized Gains	5,567	15,460
Gross Unrealized Losses	(515)
Fair Value	144,307	115,329
U.S. government agency securities [Member]
Schedule of Held-to-Maturity Securities [Line Items]
Amortized Cost	4,427
Gross Unrealized Gains
Gross Unrealized Losses	(62)
Fair Value	4,365
Obligations of states and state agencies [Member]
Schedule of Held-to-Maturity Securities [Line Items]
Amortized Cost	192,653	180,304
Gross Unrealized Gains	1,944	11,817
Gross Unrealized Losses	(5,473)	(105)
Fair Value	189,124	192,016
Municipal bonds [Member]
Schedule of Held-to-Maturity Securities [Line Items]
Amortized Cost	353,233	326,169
Gross Unrealized Gains	6,053	13,873
Gross Unrealized Losses	(5,259)	(92)
Fair Value	354,027	339,950
Obligations of states and political subdivisions [Member]
Schedule of Held-to-Maturity Securities [Line Items]
Amortized Cost	545,886	506,473
Gross Unrealized Gains	7,997	25,690
Gross Unrealized Losses	(10,732)	(197)
Fair Value	543,151	531,966
Residential mortgage-backed securities [Member]
Schedule of Held-to-Maturity Securities [Line Items]
Amortized Cost	886,043	813,647
Gross Unrealized Gains	12,609	24,824
Gross Unrealized Losses	(27,631)	(355)
Fair Value	871,021	838,116
Trust preferred securities [Member]
Schedule of Held-to-Maturity Securities [Line Items]
Amortized Cost	103,458	127,505
Gross Unrealized Gains	363	930
Gross Unrealized Losses	(12,332)	(14,778)
Fair Value	91,489	113,657
Corporate and other debt securities [Member]
Schedule of Held-to-Maturity Securities [Line Items]
Amortized Cost	52,668	52,213
Gross Unrealized Gains	4,426	6,669
Gross Unrealized Losses
Fair Value	$ 57,094	$ 58,882