Fair Value Measurements - Recurring (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Fair Value
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, Liabilities, Level 2 to Level 1 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	65,179		$ 42,817
Short-term investments	54,555		79,489
Contingent liability			666
Level 1 | Recurring
Fair Value
Cash and cash equivalents	65,179		39,051
Level 2 | Recurring
Fair Value
Cash and cash equivalents			3,766
Short-term investments	$ 54,555		79,489
Level 3 | Recurring
Fair Value
Contingent liability			$ 666