Fair Value Measurements - Narrative (Details) - USD ($) $ / shares in Units, $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities measured at fair value on a recurring basis	$ 78,543	$ 77,412
Contingent purchase consideration
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities measured at fair value on a recurring basis	78,200	$ 76,900
Current Liabilities [Member] | Contingent purchase consideration
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities measured at fair value on a recurring basis	$ 32,300
Warrants From July 2011 Debt Financing [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Number of common stock that can be purchased (shares)	28,125
Exercise price of warrants (in usd per share)	$ 72.00