Summary of Significant Accounting Policies - Short-term Investments and Fair Value of Financial Instruments (Details) - USD ($)	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Accumulated other comprehensive income	$ 295,000		$ 102,000
Realized gains	100,000	$ 0
Allowance for expected credit loss (less than)	35,000		48,000
Investments:
Total investments	59,608,000		126,605,000
Fair value of assets transferred from Level 1 to Level 2	0		0
Fair value of assets transferred from Level 2 to Level 1	0		0
Fair value of liabilities transferred from Level 1 to Level 2	0		0
Fair value of liabilities transferred from Level 2 to Level 1	0		0
Recurring basis
Cash equivalents:
Money market funds	65,624,000		18,429,000
U.S. government securities	8,000,000
Total cash equivalents	73,624,000		18,429,000
Investments:
Commercial paper	6,292,000		14,214,000
Corporate bonds			32,485,000
Asset-backed securities	6,094,000		8,100,000
U.S. government securities	53,361,000		78,082,000
Total investments	65,747,000		132,881,000
Total cash equivalents and investments	139,371,000		151,310,000
Level 1 | Recurring basis
Cash equivalents:
Money market funds	65,624,000		18,429,000
U.S. government securities	8,000,000
Total cash equivalents	73,624,000		18,429,000
Investments:
Commercial paper	0		0
Corporate bonds			0
Asset-backed securities	0		0
U.S. government securities	53,361,000		78,082,000
Total investments	53,361,000		78,082,000
Total cash equivalents and investments	126,985,000		96,511,000
Level 2 | Recurring basis
Cash equivalents:
Money market funds	0		0
U.S. government securities	0
Total cash equivalents	0		0
Investments:
Commercial paper	6,292,000		14,214,000
Corporate bonds			32,485,000
Asset-backed securities	6,094,000		8,100,000
U.S. government securities	0		0
Total investments	12,386,000		54,799,000
Total cash equivalents and investments	12,386,000		54,799,000
Level 3 | Recurring basis
Cash equivalents:
Money market funds	0		0
U.S. government securities	0
Total cash equivalents	0		0
Investments:
Commercial paper	0		0
Corporate bonds			0
Asset-backed securities	0		0
U.S. government securities	0		0
Total investments	0		0
Total cash equivalents and investments	$ 0		$ 0