Marketable Securities - Additional Information (Detail)	6 Months Ended
	Jun. 30, 2018
Debt Securities, Available-for-sale [Line Items]
Weighted-average remaining maturity period of marketable debt securities, months	1 year 6 months
Minimum
Debt Securities, Available-for-sale [Line Items]
Remaining maturities of marketable debt securities	2018-07
Maximum
Debt Securities, Available-for-sale [Line Items]
Remaining maturities of marketable debt securities	2021-08