Securities - Gross Unrealized Losses, Aggregated by Investment Category and Length of Time (Detail) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Gross Unrealized Losses, Less Than Twelve Months	$ (1,601)	$ (3,803)
Fair Value, Less Than Twelve Months	133,456	167,230
Gross Unrealized Losses, Twelve Months and Over	(10)	(7)
Fair Value, Twelve Months and Over	486	526
U.S. Treasury [Member]
Schedule of Available-for-sale Securities [Line Items]
Gross Unrealized Losses, Less Than Twelve Months	(107)	(146)
Fair Value, Less Than Twelve Months	15,717	15,745
U.S. Government Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Gross Unrealized Losses, Less Than Twelve Months	(883)	(1,795)
Fair Value, Less Than Twelve Months	67,986	77,471
Mortgage-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Gross Unrealized Losses, Less Than Twelve Months	(475)	(879)
Fair Value, Less Than Twelve Months	34,070	36,474
State and Local Governments [Member]
Schedule of Available-for-sale Securities [Line Items]
Gross Unrealized Losses, Less Than Twelve Months	(136)	(983)
Fair Value, Less Than Twelve Months	15,683	37,540
Gross Unrealized Losses, Twelve Months and Over	(10)	(7)
Fair Value, Twelve Months and Over	$ 486	$ 526