Investment Securities - Gross Unrealized Losses on Investment (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair Value	$ 464,152	$ 202,760
Less than 12 months, Unrealized Loss	(11,086)	(2,044)
12 months or more, Fair Value	0	0
12 months or more, Unrealized Loss	0	0
Total Fair Value	464,152	202,760
Total Unrealized Loss	(11,086)	(2,044)
Less than 12 months, Fair Value, held to maturity	290,481	198,978
Less than 12 months, Unrealized Loss, held to maturity	(4,390)	(2,893)
12 months or more, Fair Value, held to maturity		1,121
12 months or more, Unrealized Loss, held to maturity		(99)
Total Fair Value, held to maturity	290,481	200,099
Total Unrealized Loss, held to maturity	(4,390)	(2,992)
Obligations of U.S. Government Corporations and Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair Value	370,389	193,306
Less than 12 months, Unrealized Loss	(6,628)	(1,996)
12 months or more, Fair Value	0	0
12 months or more, Unrealized Loss	0	0
Total Fair Value	370,389	193,306
Total Unrealized Loss	(6,628)	(1,996)
Less than 12 months, Fair Value, held to maturity	280,497	198,481
Less than 12 months, Unrealized Loss, held to maturity	(4,199)	(2,882)
Total Fair Value, held to maturity	280,497	198,481
Total Unrealized Loss, held to maturity	(4,199)	(2,882)
Obligations of States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair Value	90,825	6,469
Less than 12 months, Unrealized Loss	(4,396)	(33)
12 months or more, Fair Value	0	0
12 months or more, Unrealized Loss	0	0
Total Fair Value	90,825	6,469
Total Unrealized Loss	(4,396)	(33)
Less than 12 months, Fair Value, held to maturity	9,984	497
Less than 12 months, Unrealized Loss, held to maturity	(191)	(11)
12 months or more, Fair Value, held to maturity		1,121
12 months or more, Unrealized Loss, held to maturity		(99)
Total Fair Value, held to maturity	9,984	1,618
Total Unrealized Loss, held to maturity	(191)	(110)
Marketable Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair Value	2,938	2,985
Less than 12 months, Unrealized Loss	(62)	(15)
12 months or more, Fair Value	0	0
12 months or more, Unrealized Loss	0	0
Total Fair Value	2,938	2,985
Total Unrealized Loss	$ (62)	$ (15)