Investment Securities - Age of Unrealized Losses and Fair Value of Related Securities (Detail) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Schedule of Held-to-Maturity Securities [Line Items]
Less than Twelve Months, Fair Value	$ 517,638	$ 929,411
Less than Twelve Months, Unrealized Losses	(1,148)	(6,522)
More than Twelve Months, Fair Value	243,075	241,111
More than Twelve Months, Unrealized Losses	(8,608)	(9,790)
Total, Fair Value	760,713	1,170,522
Total, Unrealized Losses	(9,756)	(16,312)
U.S. Treasury securities
Schedule of Held-to-Maturity Securities [Line Items]
Less than Twelve Months, Fair Value	360,004	548,538
Less than Twelve Months, Unrealized Losses	(19)	(1,704)
More than Twelve Months, Fair Value	0	0
More than Twelve Months, Unrealized Losses	0	0
Total, Fair Value	360,004	548,538
Total, Unrealized Losses	(19)	(1,704)
U.S. government agency securities
Schedule of Held-to-Maturity Securities [Line Items]
Less than Twelve Months, Fair Value	0	3,489
Less than Twelve Months, Unrealized Losses	0	(5)
More than Twelve Months, Fair Value	4,517	4,736
More than Twelve Months, Unrealized Losses	(13)	(13)
Total, Fair Value	4,517	8,225
Total, Unrealized Losses	(13)	(18)
Obligations of states and state agencies
Schedule of Held-to-Maturity Securities [Line Items]
Less than Twelve Months, Fair Value	1,576	24,359
Less than Twelve Months, Unrealized Losses	(6)	(67)
More than Twelve Months, Fair Value	0	0
More than Twelve Months, Unrealized Losses	0	0
Total, Fair Value	1,576	24,359
Total, Unrealized Losses	(6)	(67)
Municipal bonds
Schedule of Held-to-Maturity Securities [Line Items]
Less than Twelve Months, Fair Value	0	38,207
Less than Twelve Months, Unrealized Losses	0	(128)
More than Twelve Months, Fair Value	12,513	13,551
More than Twelve Months, Unrealized Losses	(314)	(246)
Total, Fair Value	12,513	51,758
Total, Unrealized Losses	(314)	(374)
Obligations of states and political subdivisions
Schedule of Held-to-Maturity Securities [Line Items]
Less than Twelve Months, Fair Value	1,576	62,566
Less than Twelve Months, Unrealized Losses	(6)	(195)
More than Twelve Months, Fair Value	12,513	13,551
More than Twelve Months, Unrealized Losses	(314)	(246)
Total, Fair Value	14,089	76,117
Total, Unrealized Losses	(320)	(441)
Residential mortgage-backed securities
Schedule of Held-to-Maturity Securities [Line Items]
Less than Twelve Months, Fair Value	141,851	293,615
Less than Twelve Months, Unrealized Losses	(804)	(4,147)
More than Twelve Months, Fair Value	172,436	164,010
More than Twelve Months, Unrealized Losses	(3,175)	(4,537)
Total, Fair Value	314,287	457,625
Total, Unrealized Losses	(3,979)	(8,684)
Trust preferred securities
Schedule of Held-to-Maturity Securities [Line Items]
Less than Twelve Months, Fair Value	0	0
Less than Twelve Months, Unrealized Losses	0	0
More than Twelve Months, Fair Value	8,259	8,404
More than Twelve Months, Unrealized Losses	(2,142)	(2,054)
Total, Fair Value	8,259	8,404
Total, Unrealized Losses	(2,142)	(2,054)
Corporate and other debt securities
Schedule of Held-to-Maturity Securities [Line Items]
Less than Twelve Months, Fair Value	14,207	21,203
Less than Twelve Months, Unrealized Losses	(319)	(471)
More than Twelve Months, Fair Value	31,566	36,137
More than Twelve Months, Unrealized Losses	(953)	(1,418)
Total, Fair Value	45,773	57,340
Total, Unrealized Losses	(1,272)	(1,889)
Equity securities
Schedule of Held-to-Maturity Securities [Line Items]
Less than Twelve Months, Fair Value	0	0
Less than Twelve Months, Unrealized Losses	0	0
More than Twelve Months, Fair Value	13,784	14,273
More than Twelve Months, Unrealized Losses	(2,011)	(1,522)
Total, Fair Value	13,784	14,273
Total, Unrealized Losses	$ (2,011)	$ (1,522)