Investments - Summary of Unrealized Loss Position (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	$ 31,042	$ 28,638
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	12	24
Fair Value, Less than 12 Months	18,319	15,664
Unrealized Losses, Less than 12 Months	(157)	(60)
Fair Value, 12 Months or More	11,845	0
Unrealized Losses, 12 Months or More	(49)	0
Fair Value	30,164	15,664
Unrealized Losses	(206)	(60)
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	(206)	(60)
Available-for-sale Securities	30,848	28,602
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	29,970	26,280
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	0	11
Fair Value, Less than 12 Months	17,919	14,663
Unrealized Losses, Less than 12 Months	(157)	(59)
Fair Value, 12 Months or More	11,845	0
Unrealized Losses, 12 Months or More	(49)	0
Fair Value	29,764	14,663
Unrealized Losses	(206)	(59)
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	(206)	(59)
Available-for-sale Securities	29,764	26,232
Corporate debt
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	1,072	2,358
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	12	13
Fair Value, Less than 12 Months	400	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	400	1,001
Unrealized Losses	0	(1)
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	0	(1)
Available-for-sale Securities	$ 1,084	$ 2,370