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Assets - Available-For-Sale Securities (Details) - USD ($) $ in Thousands	3 Months Ended			9 Months Ended
	Mar. 31, 2019	Sep. 30, 2018	Mar. 31, 2018	Sep. 30, 2018	Dec. 31, 2018
Total available-for-sale securities
Amortized Cost	$ 67,204				$ 48,205
Gross Unrealized Gains	11
Gross Unrealized Losses					(16)
Estimated Fair Value	67,215				48,189
Available-for-sale securities in a loss position
Estimated Fair Value					45,700
Gross Unrealized Losses					(16)
Short-term investments in a continuous loss position
Investments that had been in a continuous loss position for more than 12 months	0				0
Realized gains or losses
Realized gains	0		$ 0
Realized losses		$ 0		$ 0
U.S. treasuries
Total available-for-sale securities
Amortized Cost	51,715				37,191
Gross Unrealized Gains	11
Gross Unrealized Losses					(7)
Estimated Fair Value	51,726				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	4,964
Estimated Fair Value	4,964
Corporate debt
Total available-for-sale securities
Amortized Cost	1,999				8,525
Gross Unrealized Losses					(9)
Estimated Fair Value	1,999				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	8,526				2,489
Estimated Fair Value	$ 8,526				$ 2,489