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Assets - Available-For-Sale Securities (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Total available-for-sale securities
Amortized Cost	$ 84,466		$ 84,466		$ 48,205
Gross Unrealized Gains	29		29
Gross Unrealized Losses					(16)
Estimated Fair Value	84,495		84,495		48,189
Available-for-sale securities in a loss position
Estimated Fair Value					45,700
Gross Unrealized Losses					(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
U.S. treasuries
Total available-for-sale securities
Amortized Cost	56,877		56,877		37,191
Gross Unrealized Gains	28		28
Gross Unrealized Losses					(7)
Estimated Fair Value	56,905		56,905		37,184
Available-for-sale securities in a loss position
Estimated Fair Value					37,184
Gross Unrealized Losses					(7)
Government agency securities
Total available-for-sale securities
Amortized Cost	4,995		4,995
Estimated Fair Value	4,995		4,995
Corporate debt
Total available-for-sale securities
Amortized Cost	1,001		1,001		8,525
Gross Unrealized Losses					(9)
Estimated Fair Value	1,001		1,001		8,516
Available-for-sale securities in a loss position
Estimated Fair Value					8,516
Gross Unrealized Losses					(9)
Commercial paper
Total available-for-sale securities
Amortized Cost	21,593		21,593		2,489
Gross Unrealized Gains	1		1
Estimated Fair Value	$ 21,594		$ 21,594		$ 2,489