Fair Value Measurements - Additional Information (Detail)	6 Months Ended	12 Months Ended
	Jun. 30, 2019 USD ($) MarketableSecurity	Dec. 31, 2018 USD ($) MarketableSecurity	Jun. 30, 2018 USD ($)
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 not part of remaining contractual maturities of one year or less | MarketableSecurity	48
Marketable securities fair value held to maturity	$ 185,000,000.0
Impairment of assets	$ 0	$ 0
U.S. Corporate Bonds [Member]
Debt Instrument Fair Value Carrying Value [Line Items]
Number of marketable securities not part of remaining contractual maturities of one year or less | MarketableSecurity		2
Maximum [Member]
Debt Instrument Fair Value Carrying Value [Line Items]
Marketable securities, remaining contractual maturities	1 year	1 year
Marketable securities held, matuirity period	2 years
Minimum [Member]
Debt Instrument Fair Value Carrying Value [Line Items]
Marketable securities held, matuirity period	1 year