Investment Securities - Gross Unrealized Losses on Investment (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair Value	$ 400,146	$ 464,152
Less than 12 months, Unrealized Loss	(5,265)	(11,086)
12 months or more, Fair Value	8,905
12 months or more, Unrealized Loss	(697)
Total Fair Value	409,051	464,152
Total Unrealized Loss	(5,962)	(11,086)
Less than 12 months, Fair Value, held to maturity	136,650	290,481
Less than 12 months, Unrealized Loss, held to maturity	(1,492)	(4,390)
12 months or more, Fair Value, held to maturity	0	0
12 months or more, Unrealized Loss, held to maturity	0	0
Total Fair Value, held to maturity	136,650	290,481
Total Unrealized Loss, held to maturity	(1,492)	(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	365,252	370,389
Less than 12 months, Unrealized Loss	(4,319)	(6,628)
Total Fair Value	365,252	370,389
Total Unrealized Loss	(4,319)	(6,628)
Less than 12 months, Fair Value, held to maturity	133,453	280,497
Less than 12 months, Unrealized Loss, held to maturity	(1,442)	(4,199)
12 months or more, Fair Value, held to maturity	0	0
12 months or more, Unrealized Loss, held to maturity	0	0
Total Fair Value, held to maturity	133,453	280,497
Total Unrealized Loss, held to maturity	(1,442)	(4,199)
Obligations of States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair Value	31,937	90,825
Less than 12 months, Unrealized Loss	(903)	(4,396)
12 months or more, Fair Value	8,905
12 months or more, Unrealized Loss	(697)
Total Fair Value	40,842	90,825
Total Unrealized Loss	(1,600)	(4,396)
Less than 12 months, Fair Value, held to maturity	3,197	9,984
Less than 12 months, Unrealized Loss, held to maturity	(50)	(191)
12 months or more, Fair Value, held to maturity	0	0
12 months or more, Unrealized Loss, held to maturity	0	0
Total Fair Value, held to maturity	3,179	9,984
Total Unrealized Loss, held to maturity	(50)	(191)
Marketable Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair Value	2,957	2,938
Less than 12 months, Unrealized Loss	(43)	(62)
Total Fair Value	2,957	2,938
Total Unrealized Loss	$ (43)	$ (62)