Investment Securities - Age of Unrealized Losses and Fair Value of Related Securities Held to Maturity (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Schedule of Held-to-Maturity Securities [Line Items]
Less than Twelve Months, Fair Value	$ 112,284	$ 710,652
Less than Twelve Months, Unrealized Losses	(613)	(39,085)
More than Twelve Months, Fair Value	390,639	57,806
More than Twelve Months, Unrealized Losses	(18,701)	(12,187)
Total, Fair Value	502,923	768,458
Total, Unrealized Losses	(19,314)	(51,272)
U.S. Treasury securities [Member]
Schedule of Held-to-Maturity 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 Held-to-Maturity 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 Held-to-Maturity Securities [Line Items]
Less than Twelve Months, Fair Value	4,927	80,612
Less than Twelve Months, Unrealized Losses	(50)	(5,473)
More than Twelve Months, Fair Value	19,050	0
More than Twelve Months, Unrealized Losses	(362)	0
Total, Fair Value	23,977	80,612
Total, Unrealized Losses	(412)	(5,473)
Municipal bonds [Member]
Schedule of Held-to-Maturity 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	28,815	1,326
More than Twelve Months, Unrealized Losses	(278)	(105)
Total, Fair Value	28,815	87,314
Total, Unrealized Losses	(278)	(5,259)
Obligations of states and political subdivisions [Member]
Schedule of Held-to-Maturity Securities [Line Items]
Less than Twelve Months, Fair Value	4,927	166,600
Less than Twelve Months, Unrealized Losses	(50)	(10,627)
More than Twelve Months, Fair Value	47,865	1,326
More than Twelve Months, Unrealized Losses	(640)	(105)
Total, Fair Value	52,792	167,926
Total, Unrealized Losses	(690)	(10,732)
Residential mortgage-backed securities [Member]
Schedule of Held-to-Maturity Securities [Line Items]
Less than Twelve Months, Fair Value	107,357	465,400
Less than Twelve Months, Unrealized Losses	(563)	(27,631)
More than Twelve Months, Fair Value	276,580	0
More than Twelve Months, Unrealized Losses	(5,681)	0
Total, Fair Value	383,937	465,400
Total, Unrealized Losses	(6,244)	(27,631)
Trust preferred securities [Member]
Schedule of Held-to-Maturity 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	66,194	56,480
More than Twelve Months, Unrealized Losses	(12,380)	(12,082)
Total, Fair Value	66,194	66,230
Total, Unrealized Losses	$ (12,380)	$ (12,332)