Gross Unrealized Losses and Fair Value, Aggregated by Investment Category and Length of Time that Individual Securities have been in a Continuous Unrealized Loss Position (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Gain (Loss) on Investments [Line Items]
Less than 12 Months, Fair Value	$ 48,768
Less than 12 Months, Unrealized Losses	(578)
12 Months or More, Fair Value	35,601
12 Months or More, Unrealized Losses	(649)
Total, Fair Value	84,369
Total, Unrealized Losses	(1,227)
U.S. government agencies
Gain (Loss) on Investments [Line Items]
Less than 12 Months, Fair Value	8,830	$ 12,190
Less than 12 Months, Unrealized Losses	(171)	(59)
12 Months or More, Fair Value	35,601	32,769
12 Months or More, Unrealized Losses	(649)	(231)
Total, Fair Value	44,431	44,959
Total, Unrealized Losses	(820)	$ (290)
State and political subdivisions
Gain (Loss) on Investments [Line Items]
Less than 12 Months, Fair Value	39,938
Less than 12 Months, Unrealized Losses	(407)
12 Months or More, Fair Value	0
12 Months or More, Unrealized Losses	0
Total, Fair Value	39,938
Total, Unrealized Losses	$ (407)