Assets - Available-For-Sale Securities (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Total available-for-sale securities
Amortized Cost	$ 79,194		$ 115,219
Gross Unrealized Gains	252		38
Gross Unrealized Losses	(17)		(5)
Estimated Fair Value	79,429		115,252
Available-for-sale securities in a loss position
Estimated Fair Value	7,015		28,945
Gross Unrealized Losses	(17)		(5)
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	52,285		105,096
Gross Unrealized Gains	222		38
Gross Unrealized Losses			(4)
Estimated Fair Value	52,507		105,130
Available-for-sale securities in a loss position
Estimated Fair Value			22,943
Gross Unrealized Losses			(4)
Government agency securities
Total available-for-sale securities
Amortized Cost	5,947	1,139
Gross Unrealized Gains	29
Estimated Fair Value	5,976	$ 1,139
Corporate debt
Total available-for-sale securities
Amortized Cost	7,032		6,003
Gross Unrealized Losses	(17)		(1)
Estimated Fair Value	7,015		6,002
Available-for-sale securities in a loss position
Estimated Fair Value	7,015		6,002
Gross Unrealized Losses	(17)		(1)
Commercial paper
Total available-for-sale securities
Amortized Cost	13,930		2,981
Gross Unrealized Gains	1
Estimated Fair Value	$ 13,931		$ 2,981