Fair Value Measurements - Liabilities (Details) - Recurring - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Fair Value Measurements
Acquisition-related contingent consideration - short-term	$ 8,540	$ 43,397
Acquisition-related contingent consideration - long-term	10,200	7,800
Liabilities	18,740	51,197
Level 3
Fair Value Measurements
Acquisition-related contingent consideration - short-term	8,540	43,397
Acquisition-related contingent consideration - long-term	10,200	7,800
Liabilities	$ 18,740	$ 51,197