Fair Value Measurement - Assets and Liabilities Measured on a Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Liabilities:
Contingent consideration		$ 19,283
Quoted Prices in Active Markets for Identical Assets (Level 1)
Liabilities:
Contingent consideration		0
Significant Other Observable Inputs (Level 2)
Liabilities:
Contingent consideration		0
Significant Unobservable Inputs (Level 3)
Liabilities:
Contingent consideration	$ 0	$ 19,283	$ 22,162