Investment Securities - Age of Unrealized Losses and Fair Value of Related Securities (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Schedule of Held-to-Maturity Securities [Line Items]
Less than Twelve Months, Fair Value	$ 96,037	$ 559,333
Less than Twelve Months, Unrealized Losses	(892)	(38,574)
More than Twelve Months, Fair Value	496,064	56,475
More than Twelve Months, Unrealized Losses	(20,274)	(7,688)
Total, Fair Value	592,101	615,808
Total, Unrealized Losses	(21,166)	(46,262)
U.S. Treasury Securities [Member]
Schedule of Held-to-Maturity Securities [Line Items]
Less than Twelve Months, Fair Value	0	84,665
Less than Twelve Months, Unrealized Losses	0	(15,170)
More than Twelve Months, Fair Value	92,996	0
More than Twelve Months, Unrealized Losses	(6,833)	0
Total, Fair Value	92,996	84,665
Total, Unrealized Losses	(6,833)	(15,170)
U.S. Government Agency Securities [Member]
Schedule of Held-to-Maturity Securities [Line Items]
Less than Twelve Months, Fair Value	0	26,402
Less than Twelve Months, Unrealized Losses	0	(703)
More than Twelve Months, Fair Value	20,574	0
More than Twelve Months, Unrealized Losses	(246)	0
Total, Fair Value	20,574	26,402
Total, Unrealized Losses	(246)	(703)
Obligations of States and State Agencies [Member]
Schedule of Held-to-Maturity Securities [Line Items]
Less than Twelve Months, Fair Value	0	10,598
Less than Twelve Months, Unrealized Losses	0	(798)
More than Twelve Months, Fair Value	11,111	0
More than Twelve Months, Unrealized Losses	(108)	0
Total, Fair Value	11,111	10,598
Total, Unrealized Losses	(108)	(798)
Municipal Bonds [Member]
Schedule of Held-to-Maturity Securities [Line Items]
Less than Twelve Months, Fair Value	10,939	13,461
Less than Twelve Months, Unrealized Losses	(323)	(1,365)
More than Twelve Months, Fair Value	14,495	0
More than Twelve Months, Unrealized Losses	(284)	0
Total, Fair Value	25,434	13,461
Total, Unrealized Losses	(607)	(1,365)
Obligations of States and Political Subdivisions [Member]
Schedule of Held-to-Maturity Securities [Line Items]
Less than Twelve Months, Fair Value	10,939	24,059
Less than Twelve Months, Unrealized Losses	(323)	(2,163)
More than Twelve Months, Fair Value	25,606	0
More than Twelve Months, Unrealized Losses	(392)	0
Total, Fair Value	36,545	24,059
Total, Unrealized Losses	(715)	(2,163)
Residential Mortgage-Backed Securities [Member]
Schedule of Held-to-Maturity Securities [Line Items]
Less than Twelve Months, Fair Value	59,755	368,306
Less than Twelve Months, Unrealized Losses	(422)	(18,434)
More than Twelve Months, Fair Value	300,773	24,734
More than Twelve Months, Unrealized Losses	(8,945)	(2,285)
Total, Fair Value	360,528	393,040
Total, Unrealized Losses	(9,367)	(20,719)
Trust Preferred Securities [Member]
Schedule of Held-to-Maturity Securities [Line Items]
Less than Twelve Months, Fair Value	0	2,024
Less than Twelve Months, Unrealized Losses	0	(25)
More than Twelve Months, Fair Value	12,557	15,022
More than Twelve Months, Unrealized Losses	(2,414)	(4,206)
Total, Fair Value	12,557	17,046
Total, Unrealized Losses	(2,414)	(4,231)
Corporate and Other Debt Securities [Member]
Schedule of Held-to-Maturity Securities [Line Items]
Less than Twelve Months, Fair Value	25,297	53,654
Less than Twelve Months, Unrealized Losses	(145)	(2,073)
More than Twelve Months, Fair Value	28,499	2,471
More than Twelve Months, Unrealized Losses	(859)	(30)
Total, Fair Value	53,796	56,125
Total, Unrealized Losses	(1,004)	(2,103)
Equity Securities [Member]
Schedule of Held-to-Maturity Securities [Line Items]
Less than Twelve Months, Fair Value	46	223
Less than Twelve Months, Unrealized Losses	(2)	(6)
More than Twelve Months, Fair Value	15,059	14,248
More than Twelve Months, Unrealized Losses	(585)	(1,167)
Total, Fair Value	15,105	14,471
Total, Unrealized Losses	$ (587)	$ (1,173)