Assets - Available-For-Sale Securities (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2021	Dec. 31, 2020
Total available-for-sale securities
Amortized Cost		$ 214,659	$ 189,791
Gross Unrealized Gains		30	15
Gross Unrealized Losses		(54)	(35)
Estimated Fair Value		214,635	189,771
Available-for-sale securities in a loss position
Estimated Fair Value		110,344	52,545
Gross Unrealized Losses		(54)	(35)
Investments that had been in a continuous loss position for more than 12 months		0	0
Contractual maturities - Amortized Cost
Amortized Cost, Due in one year or less		152,949
Amortized Cost, Due after one year through two years		55,693
Amortized Cost, Due after two years through three years		6,017
Amortized Cost		214,659	189,791
Contractual maturities - Estimated Fair Value
Estimated Fair Value, Due in one year or less		152,932
Estimated Fair Value, Due after one year through two years		55,679
Estimated Fair Value, Due after two years through three years		6,024
Available-for-sale Securities, Debt Securities, Total		214,635	189,771
Realized gains or losses
Realized gains	$ 0
Realized losses	$ 0
U.S. treasuries
Total available-for-sale securities
Amortized Cost		124,140	149,206
Gross Unrealized Gains		4	14
Gross Unrealized Losses		(15)	(1)
Estimated Fair Value		124,129	149,219
Available-for-sale securities in a loss position
Estimated Fair Value		61,520	19,991
Gross Unrealized Losses		(15)	(1)
Contractual maturities - Amortized Cost
Amortized Cost		124,140	149,206
Contractual maturities - Estimated Fair Value
Available-for-sale Securities, Debt Securities, Total		124,129	149,219
Government agency securities
Total available-for-sale securities
Amortized Cost		7,258
Gross Unrealized Losses		(3)
Estimated Fair Value		7,255
Available-for-sale securities in a loss position
Estimated Fair Value		7,255
Gross Unrealized Losses		(3)
Contractual maturities - Amortized Cost
Amortized Cost		7,258
Contractual maturities - Estimated Fair Value
Available-for-sale Securities, Debt Securities, Total		7,255
Corporate debt
Total available-for-sale securities
Amortized Cost		65,676	32,588
Gross Unrealized Gains		26
Gross Unrealized Losses		(36)	(34)
Estimated Fair Value		65,666	32,554
Available-for-sale securities in a loss position
Estimated Fair Value		41,569	32,554
Gross Unrealized Losses		(36)	(34)
Contractual maturities - Amortized Cost
Amortized Cost		65,676	32,588
Contractual maturities - Estimated Fair Value
Available-for-sale Securities, Debt Securities, Total		65,666	32,554
Commercial paper
Total available-for-sale securities
Amortized Cost		17,585	7,997
Gross Unrealized Gains			1
Estimated Fair Value		17,585	7,998
Contractual maturities - Amortized Cost
Amortized Cost		17,585	7,997
Contractual maturities - Estimated Fair Value
Available-for-sale Securities, Debt Securities, Total		$ 17,585	$ 7,998