Investment Securities - Age of Unrealized Losses and Fair Value of Related Securities Held to Maturity (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Less than Twelve Months, Fair Value	$ 149,262	$ 710,652
Less than Twelve Months, Unrealized Losses	(1,781)	(39,085)
More than Twelve Months, Fair Value	429,936	57,806
More than Twelve Months, Unrealized Losses	(24,685)	(12,187)
Total, Fair Value	579,198	768,458
Total, Unrealized Losses	(26,466)	(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	10,364	80,612
Less than Twelve Months, Unrealized Losses	(12)	(5,473)
More than Twelve Months, Fair Value	46,593	0
More than Twelve Months, Unrealized Losses	(1,279)	0
Total, Fair Value	56,957	80,612
Total, Unrealized Losses	(1,291)	(5,473)
Municipal Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than Twelve Months, Fair Value	2,276	85,988
Less than Twelve Months, Unrealized Losses	(9)	(5,154)
More than Twelve Months, Fair Value	55,006	1,326
More than Twelve Months, Unrealized Losses	(1,206)	(105)
Total, Fair Value	57,282	87,314
Total, Unrealized Losses	(1,215)	(5,259)
Obligations of States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than Twelve Months, Fair Value	12,640	166,600
Less than Twelve Months, Unrealized Losses	(21)	(10,627)
More than Twelve Months, Fair Value	101,599	1,326
More than Twelve Months, Unrealized Losses	(2,485)	(105)
Total, Fair Value	114,239	167,926
Total, Unrealized Losses	(2,506)	(10,732)
Residential Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than Twelve Months, Fair Value	126,574	465,400
Less than Twelve Months, Unrealized Losses	(1,558)	(27,631)
More than Twelve Months, Fair Value	272,328	0
More than Twelve Months, Unrealized Losses	(9,641)	0
Total, Fair Value	398,902	465,400
Total, Unrealized Losses	(11,199)	(27,631)
Trust Preferred Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than Twelve Months, Fair Value	9,800	9,750
Less than Twelve Months, Unrealized Losses	(200)	(250)
More than Twelve Months, Fair Value	56,009	56,480
More than Twelve Months, Unrealized Losses	(12,559)	(12,082)
Total, Fair Value	65,809	66,230
Total, Unrealized Losses	(12,759)	(12,332)
Corporate and Other Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than Twelve Months, Fair Value	248
Less than Twelve Months, Unrealized Losses	(2)
More than Twelve Months, Fair Value	0
More than Twelve Months, Unrealized Losses	0
Total, Fair Value	248
Total, Unrealized Losses	$ (2)