Assets - Available-For-Sale Securities (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Total available-for-sale securities
Amortized Cost	$ 112,191		$ 115,219
Gross Unrealized Gains	96		38
Gross Unrealized Losses	(8)		(5)
Estimated Fair Value	112,279		115,252
Available-for-sale securities in a loss position
Estimated Fair Value	59,360		28,945
Gross Unrealized Losses	(8)		(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	77,692		105,096
Gross Unrealized Gains	58		38
Gross Unrealized Losses	(4)		(4)
Estimated Fair Value	77,746		105,130
Available-for-sale securities in a loss position
Estimated Fair Value	55,320		22,943
Gross Unrealized Losses	(4)		(4)
Government agency securities
Total available-for-sale securities
Amortized Cost	5,971		1,139
Gross Unrealized Gains	24
Estimated Fair Value	5,995		1,139
Corporate debt
Total available-for-sale securities
Amortized Cost	12,069		6,003
Gross Unrealized Gains	13
Gross Unrealized Losses	(4)		(1)
Estimated Fair Value	12,078		6,002
Available-for-sale securities in a loss position
Estimated Fair Value	4,040		6,002
Gross Unrealized Losses	(4)		(1)
Commercial paper
Total available-for-sale securities
Amortized Cost	16,459		2,981
Gross Unrealized Gains	1
Estimated Fair Value	$ 16,460		$ 2,981