Investments - Summary of Unrealized Loss Position (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than 12 Months	$ 24,644	$ 15,664
Unrealized Losses, Less than 12 Months	(60)	(60)
Fair Value, 12 Months or More	7,364	0
Unrealized Losses, 12 Months or More	34	0
Fair Value	32,008	15,664
Unrealized Losses	(94)	(60)
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	24,444	14,663
Unrealized Losses, Less than 12 Months	(60)	(59)
Fair Value, 12 Months or More	7,364	0
Unrealized Losses, 12 Months or More	34	0
Fair Value	31,808	14,663
Unrealized Losses	(94)	(59)
Corporate debt
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than 12 Months	200	1,001
Unrealized Losses, Less than 12 Months	0	(1)
Fair Value, 12 Months or More	0	0
Unrealized Losses, 12 Months or More	0	0
Fair Value	200	1,001
Unrealized Losses	$ 0	$ (1)