Investments (Details) - USD ($)	Jun. 30, 2019	Dec. 31, 2018
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	107,307,000	90,974,000
Unrealized Gains	50,000	0
Unrealized Losses	(5,000)	(52,000)
Fair Value	107,352,000	90,922,000
Long-term Debt
Debt Securities, Available-for-sale [Line Items]
Cost	6,110,000
Unrealized Gains	6,000
Unrealized Losses	0
Fair Value	6,116,000
Commercial paper | Short-term Debt
Debt Securities, Available-for-sale [Line Items]
Cost	17,384,000	27,898,000
Unrealized Gains	0	0
Unrealized Losses	0	0
Fair Value	17,384,000	27,898,000
Corporate bonds | Short-term Debt
Debt Securities, Available-for-sale [Line Items]
Cost	20,208,000	28,043,000
Unrealized Gains	14,000	0
Unrealized Losses	(5,000)	(31,000)
Fair Value	20,217,000	28,012,000
Corporate bonds | Long-term Debt
Debt Securities, Available-for-sale [Line Items]
Cost	3,019,000
Unrealized Gains	6,000
Unrealized Losses	0
Fair Value	3,025,000
Asset-backed securities | Short-term Debt
Debt Securities, Available-for-sale [Line Items]
Cost	12,438,000	17,074,000
Unrealized Gains	5,000	0
Unrealized Losses	0	(19,000)
Fair Value	12,443,000	17,055,000
Asset-backed securities | Long-term Debt
Debt Securities, Available-for-sale [Line Items]
Cost	3,091,000
Unrealized Gains	0
Unrealized Losses	0
Fair Value	3,091,000
U.S. government securities | Short-term Debt
Debt Securities, Available-for-sale [Line Items]
Cost	57,277,000	17,959,000
Unrealized Gains	31,000	0
Unrealized Losses	0	(2,000)
Fair Value	$ 57,308,000	$ 17,957,000