Investments (Details) - USD ($)	Jun. 30, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Investments with maturity greater than one year	$ 0	$ 0
Short-term Debt
Debt Securities, Available-for-sale [Line Items]
Cost	31,168,000	126,522,000
Unrealized Gains	154,000	86,000
Unrealized Losses	0	(3,000)
Fair Value	31,322,000	126,605,000
Long-term Debt
Debt Securities, Available-for-sale [Line Items]
Cost		6,258,000
Unrealized Gains		18,000
Unrealized Losses		0
Fair Value		6,276,000
Commercial paper | Short-term Debt
Debt Securities, Available-for-sale [Line Items]
Cost		14,214,000
Unrealized Gains		0
Unrealized Losses		0
Fair Value		14,214,000
Corporate bonds | Short-term Debt
Debt Securities, Available-for-sale [Line Items]
Cost		26,194,000
Unrealized Gains		16,000
Unrealized Losses		(2,000)
Fair Value		26,208,000
Corporate bonds | Long-term Debt
Debt Securities, Available-for-sale [Line Items]
Cost		6,258,000
Unrealized Gains		18,000
Unrealized Losses		0
Fair Value		6,276,000
Asset-backed securities | Short-term Debt
Debt Securities, Available-for-sale [Line Items]
Cost	6,100,000	8,097,000
Unrealized Gains	12,000	5,000
Unrealized Losses	0	(1,000)
Fair Value	6,112,000	8,101,000
U.S. government securities | Short-term Debt
Debt Securities, Available-for-sale [Line Items]
Cost	25,068,000	78,017,000
Unrealized Gains	142,000	65,000
Unrealized Losses	0	0
Fair Value	$ 25,210,000	$ 78,082,000