Investment Securities - Age of Unrealized Losses and Fair Value of Related Securities Held to Maturity (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Less than Twelve Months, Fair Value	$ 151,897	$ 710,652
Less than Twelve Months, Unrealized Losses	(884)	(39,085)
More than Twelve Months, Fair Value	469,709	57,806
More than Twelve Months, Unrealized Losses	(24,738)	(12,187)
Total, Fair Value	621,606	768,458
Total, Unrealized Losses	(25,622)	(51,272)
U.S. Treasury Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than Twelve Months, Fair Value		64,537
Less than Twelve Months, Unrealized Losses		(515)
More than Twelve Months, Fair Value		0
More than Twelve Months, Unrealized Losses		0
Total, Fair Value		64,537
Total, Unrealized Losses		(515)
U.S. Government Agency Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than Twelve Months, Fair Value		4,365
Less than Twelve Months, Unrealized Losses		(62)
More than Twelve Months, Fair Value		0
More than Twelve Months, Unrealized Losses		0
Total, Fair Value		4,365
Total, Unrealized Losses		(62)
Obligations of States and State Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than Twelve Months, Fair Value	16,384	80,612
Less than Twelve Months, Unrealized Losses	(30)	(5,473)
More than Twelve Months, Fair Value	24,744	0
More than Twelve Months, Unrealized Losses	(389)	0
Total, Fair Value	41,128	80,612
Total, Unrealized Losses	(419)	(5,473)
Municipal Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than Twelve Months, Fair Value	0	85,988
Less than Twelve Months, Unrealized Losses	0	(5,154)
More than Twelve Months, Fair Value	51,368	1,326
More than Twelve Months, Unrealized Losses	(597)	(105)
Total, Fair Value	51,368	87,314
Total, Unrealized Losses	(597)	(5,259)
Obligations of States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than Twelve Months, Fair Value	16,384	166,600
Less than Twelve Months, Unrealized Losses	(30)	(10,627)
More than Twelve Months, Fair Value	76,112	1,326
More than Twelve Months, Unrealized Losses	(986)	(105)
Total, Fair Value	92,496	167,926
Total, Unrealized Losses	(1,016)	(10,732)
Residential Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than Twelve Months, Fair Value	135,266	465,400
Less than Twelve Months, Unrealized Losses	(851)	(27,631)
More than Twelve Months, Fair Value	327,191	0
More than Twelve Months, Unrealized Losses	(11,587)	0
Total, Fair Value	462,457	465,400
Total, Unrealized Losses	(12,438)	(27,631)
Trust Preferred Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than Twelve Months, Fair Value	0	9,750
Less than Twelve Months, Unrealized Losses	0	(250)
More than Twelve Months, Fair Value	66,406	56,480
More than Twelve Months, Unrealized Losses	(12,165)	(12,082)
Total, Fair Value	66,406	66,230
Total, Unrealized Losses	(12,165)	(12,332)
Corporate and Other Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than Twelve Months, Fair Value	247
Less than Twelve Months, Unrealized Losses	(3)
More than Twelve Months, Fair Value	0
More than Twelve Months, Unrealized Losses	0
Total, Fair Value	247
Total, Unrealized Losses	$ (3)