Fair Value Measurements - Recurring (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Fair Value
Short-term investments	$ 2,932		$ 2,711
Fair Value, Assets, Level 1 to Level 2 Transfers, Amount	0	$ 0
Fair Value, Assets, Level 2 to Level 1 Transfers, Amount	0	0
Fair Value, Liabilities, Level 1 to Level 2 Transfers, Amount	0	0
Fair value assets level 3 net transfers	0	0
Fair value liabilities level 3 net transfers	0	$ 0
Recurring
Fair Value
Cash and cash equivalents	1,308,843		514,353
Short-term investments	2,932		2,711
Contingent liability	4,990		4,769
Level 1 | Recurring
Fair Value
Cash and cash equivalents	1,308,843		514,353
Short-term investments	0		0
Contingent liability	0		0
Level 2 | Recurring
Fair Value
Cash and cash equivalents	0		0
Short-term investments	2,932		2,711
Contingent liability	0		0
Level 3 | Recurring
Fair Value
Cash and cash equivalents	0		0
Short-term investments	0		0
Contingent liability	$ 4,990		$ 4,769