Fair Value Measurements - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2017 USD ($) MarketableSecurity	Dec. 31, 2016 USD ($) MarketableSecurity
Debt Instrument Fair Value Carrying Value [Line Items]
Transfers among the Level 1, Level 2 and Level 3 categories	$ 0	$ 0
Number of marketable securities loss position due to fluctuations in interest rates | MarketableSecurity	11	27
Impairment of assets	$ 0	$ 0
Maximum [Member]
Debt Instrument Fair Value Carrying Value [Line Items]
Marketable securities, remaining contractual maturities	1 year