Investments - Summary of Unrealized Loss Position (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	$ 22,352	$ 31,042
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	7	12
Fair Value, Less than 12 Months	6,323	18,319
Unrealized Losses, Less than 12 Months	(6)	(157)
Fair Value, 12 Months or More	12,734	11,845
Unrealized Losses, 12 Months or More	(216)	(49)
Fair Value	19,057	30,164
Unrealized Losses	(222)	(206)
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	(222)	(206)
Available-for-sale Securities	22,137	30,848
U.S. Treasury securities, obligations of U.S. Government agencies and Municipals
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	21,729	29,970
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	7	0
Fair Value, Less than 12 Months	5,866	17,919
Unrealized Losses, Less than 12 Months	(6)	(157)
Fair Value, 12 Months or More	12,634	11,845
Unrealized Losses, 12 Months or More	(216)	(49)
Fair Value	18,500	29,764
Unrealized Losses	(222)	(206)
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	(222)	(206)
Available-for-sale Securities	21,514	29,764
Corporate debt
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	623	1,072
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	0	12
Fair Value, Less than 12 Months	457	400
Unrealized Losses, Less than 12 Months	0	0
Fair Value, 12 Months or More	100	0
Unrealized Losses, 12 Months or More	0	0
Fair Value	557	400
Unrealized Losses	0	0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	0	0
Available-for-sale Securities	$ 623	$ 1,084