Investments - Summary of Unrealized Loss Position (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than 12 Months	$ 0	$ 6,323
Unrealized Losses, Less than 12 Months	0	(6)
Fair Value, 12 Months or More	11,456	12,734
Unrealized Losses, 12 Months or More	(134)	(216)
Fair Value	11,456	19,057
Unrealized Losses	(134)	(222)
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	0	5,866
Unrealized Losses, Less than 12 Months	0	(6)
Fair Value, 12 Months or More	11,456	12,634
Unrealized Losses, 12 Months or More	(134)	(216)
Fair Value	11,456	18,500
Unrealized Losses	$ (134)	(222)
Corporate debt
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than 12 Months		457
Unrealized Losses, Less than 12 Months		0
Fair Value, 12 Months or More		100
Unrealized Losses, 12 Months or More		0
Fair Value		557
Unrealized Losses		$ 0