Investments - Summary of Unrealized Loss Position (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than 12 Months	$ 8,857	$ 18,319
Unrealized Losses, Less than 12 Months	(106)	(157)
Fair Value, 12 Months or More	12,727	11,845
Unrealized Losses, 12 Months or More	(358)	(49)
Fair Value	21,584	30,164
Unrealized Losses	(464)	(206)
U.S. Treasury securities, obligations of U.S. Government agencies and Municipalities
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than 12 Months	8,237	17,919
Unrealized Losses, Less than 12 Months	(104)	(157)
Fair Value, 12 Months or More	12,727	11,845
Unrealized Losses, 12 Months or More	(358)	(49)
Fair Value	20,964	29,764
Unrealized Losses	(462)	(206)
Corporate debt
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than 12 Months	620	400
Unrealized Losses, Less than 12 Months	(2)	0
Fair Value, 12 Months or More	0	0
Unrealized Losses, 12 Months or More	0	0
Fair Value	620	400
Unrealized Losses	$ (2)	$ 0