Short-term Investments - Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Amortized cost	$ 103,520	$ 117,159
Gross unrealized gains	225	147
Gross unrealized losses	(141)	(2)
Fair value	103,604	117,304
Debt securities and equity securities, amortized cost	114,038	127,052
Debt securities and equity securities, unrealized gains	225	772
Debt securities and equity securities, unrealized losses	(1,501)	(2)
Short-term investments	112,762	127,822
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized cost	68,049	78,243
Gross unrealized gains	6	89
Gross unrealized losses	(131)	(2)
Fair value	67,924	78,330
U.S. government-related debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized cost	26,519	26,966
Gross unrealized gains	213	37
Gross unrealized losses	0	0
Fair value	26,732	27,003
Asset-backed Securities
Debt Securities, Available-for-sale [Line Items]
Amortized cost	8,952	11,950
Gross unrealized gains	6	21
Gross unrealized losses	(10)	0
Fair value	8,948	11,971
Corporate Equity Securities
Debt Securities, Available-for-sale [Line Items]
Equity securities, Amortized cost	10,518	9,893
Equity securities, unrealized gain	0	625
Equity securities, unrealized loss	(1,360)	0
Equity securities, fair value	$ 9,158	$ 10,518