Investments (Details) - USD ($)	Mar. 31, 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	59,413,000	126,522,000
Unrealized Gains	202,000	86,000
Unrealized Losses	(7,000)	(3,000)
Fair Value	59,608,000	126,605,000
Long-term Debt
Debt Securities, Available-for-sale [Line Items]
Cost	6,047,000	6,258,000
Unrealized Gains	92,000	18,000
Unrealized Losses	0	0
Fair Value	6,139,000	6,276,000
Commercial paper \| Short-term Debt
Debt Securities, Available-for-sale [Line Items]
Cost	6,292,000	14,214,000
Unrealized Gains	0	0
Unrealized Losses	0	0
Fair Value	6,292,000	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	1,000	5,000
Unrealized Losses	(7,000)	(1,000)
Fair Value	6,094,000	8,101,000
U.S. government securities \| Short-term Debt
Debt Securities, Available-for-sale [Line Items]
Cost	47,021,000	78,017,000
Unrealized Gains	201,000	65,000
Unrealized Losses	0	0
Fair Value	47,222,000	$ 78,082,000
U.S. government securities \| Long-term Debt
Debt Securities, Available-for-sale [Line Items]
Cost	6,047,000
Unrealized Gains	92,000
Unrealized Losses	0
Fair Value	$ 6,139,000

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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