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, 2013	Dec. 31, 2012
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 1,766,947	$ 1,599,707
Gross Unrealized Gains	38,602	73,573
Gross Unrealized Losses	(41,368)	(15,330)
Fair Value	1,764,181	1,657,950
U.S. Treasury securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	99,792	99,869
Gross Unrealized Gains	8,942	15,460
Fair Value	108,734	115,329
Obligations of states and political subdivisions [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	542,154	506,473
Gross Unrealized Gains	10,120	25,690
Gross Unrealized Losses	(9,660)	(197)
Fair Value	542,614	531,966
Residential mortgage-backed securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	969,341	813,647
Gross Unrealized Gains	13,459	24,824
Gross Unrealized Losses	(19,225)	(355)
Fair Value	963,575	838,116
Trust preferred securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	103,453	127,505
Gross Unrealized Gains	377	930
Gross Unrealized Losses	(12,482)	(14,778)
Fair Value	91,348	113,657
Corporate and other debt securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	52,207	52,213
Gross Unrealized Gains	5,704	6,669
Gross Unrealized Losses	(1)
Fair Value	$ 57,910	$ 58,882