Assets - Available-For-Sale Securities (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Total available-for-sale securities
Amortized Cost	$ 95,680		$ 95,680		$ 48,205
Gross Unrealized Gains	10		10
Gross Unrealized Losses	(18)		(18)		(16)
Estimated Fair Value	95,672		95,672		48,189
Available-for-sale securities in a loss position
Estimated Fair Value	95,672		95,672		45,700
Gross Unrealized Losses	(18)		(18)		(16)
Investments that had been in a continuous loss position for more than 12 months	0		0		0
Realized gains or losses
Realized gains	0	$ 0	0	$ 0
Realized losses	0	$ 0	0	$ 0
U.S. treasuries
Total available-for-sale securities
Amortized Cost	81,733		81,733		37,191
Gross Unrealized Gains	10		10
Gross Unrealized Losses	(16)		(16)		(7)
Estimated Fair Value	81,727		81,727		37,184
Available-for-sale securities in a loss position
Estimated Fair Value	81,727		81,727		37,184
Gross Unrealized Losses	(16)		(16)		(7)
Corporate debt
Total available-for-sale securities
Amortized Cost	5,007		5,007		8,525
Gross Unrealized Losses	(1)		(1)		(9)
Estimated Fair Value	5,006		5,006		8,516
Available-for-sale securities in a loss position
Estimated Fair Value	5,006		5,006		8,516
Gross Unrealized Losses	(1)		(1)		(9)
Commercial paper
Total available-for-sale securities
Amortized Cost	8,940		8,940		2,489
Gross Unrealized Losses	(1)		(1)
Estimated Fair Value	8,939		8,939		$ 2,489
Available-for-sale securities in a loss position
Estimated Fair Value	8,939		8,939
Gross Unrealized Losses	$ (1)		$ (1)