Available-for-Sale Marketable Securities - Additional Information (Detail)	9 Months Ended	12 Months Ended
	Sep. 30, 2019 USD ($) position	Dec. 31, 2018 USD ($) position
Investments Debt And Equity Securities [Abstract]
Number of available-for-sale marketable securities in unrealized loss positions | position	23	69
Available-for-sale marketable securities, continuous unrealized loss position, 12 Months or Greater, Gross Unrealized Losses | $	$ 0	$ 0
Total number of positions | position	84	84
Other-than-temporary impairments on securities | $	$ 0	$ 0