Investment Securities - Gross Unrealized Losses on Investment (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair Value	$ 289,271	$ 464,152
Less than 12 months, Unrealized Loss	(2,211)	(11,086)
12 months or more, Fair Value	186,361
12 months or more, Unrealized Loss	(3,781)
Total Fair Value	475,632	464,152
Total Unrealized Loss	(5,992)	(11,086)
Less than 12 months, Fair Value, held to maturity	94,505	290,481
Less than 12 months, Unrealized Loss, held to maturity	(574)	(4,390)
12 months or more, Fair Value, held to maturity	98,004
12 months or more, Unrealized Loss, held to maturity	(1,352)
Total Fair Value, held to maturity	192,509	290,481
Total Unrealized Loss, held to maturity	(1,926)	(4,390)
Obligations of U.S. Government Corporations and Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair Value	284,367	370,389
Less than 12 months, Unrealized Loss	(2,176)	(6,628)
12 months or more, Fair Value	166,338
12 months or more, Unrealized Loss	(3,425)
Total Fair Value	450,705	370,389
Total Unrealized Loss	(5,601)	(6,628)
Less than 12 months, Fair Value, held to maturity	93,017	280,497
Less than 12 months, Unrealized Loss, held to maturity	(567)	(4,199)
12 months or more, Fair Value, held to maturity	95,367
12 months or more, Unrealized Loss, held to maturity	(1,322)
Total Fair Value, held to maturity	188,384	280,497
Total Unrealized Loss, held to maturity	(1,889)	(4,199)
Obligations of States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair Value	4,904	90,825
Less than 12 months, Unrealized Loss	(35)	(4,396)
12 months or more, Fair Value	17,085
12 months or more, Unrealized Loss	(294)
Total Fair Value	21,989	90,825
Total Unrealized Loss	(329)	(4,396)
Less than 12 months, Fair Value, held to maturity	1,488	9,984
Less than 12 months, Unrealized Loss, held to maturity	(7)	(191)
12 months or more, Fair Value, held to maturity	2,637
12 months or more, Unrealized Loss, held to maturity	(30)
Total Fair Value, held to maturity	4,125	9,984
Total Unrealized Loss, held to maturity	(37)	(191)
Marketable Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair Value		2,938
Less than 12 months, Unrealized Loss		(62)
12 months or more, Fair Value	2,938
12 months or more, Unrealized Loss	(62)
Total Fair Value	2,938	2,938
Total Unrealized Loss	$ (62)	$ (62)