Fair Value Measurements - Additional Information (Detail)	6 Months Ended
	Jun. 30, 2018
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Maximum maturity period of cash and cash equivalents	90 days
Maximum maturity period of short-term investments	1 year
Maximum [Member]
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Maturity period of securities	1 year