Fair Value Measurements - Liabilities (Details) - Recurring - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Fair Value Measurements
Acquisition-related contingent consideration - short-term	$ 45,304	$ 1,640
Acquisition-related contingent consideration - long-term		31,789
Liabilities	45,304	33,429
Level 3
Fair Value Measurements
Acquisition-related contingent consideration - short-term	45,304	1,640
Acquisition-related contingent consideration - long-term		31,789
Liabilities	$ 45,304	$ 33,429