Fair Value Measurements - Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Liabilities:
Contingent purchase price	$ 0	$ 14,655
Fair Value, Measurements, Recurring
Assets:
Cash equivalents	12,237	12,100
Short-term investment	13,089	0
Total assets at fair value	25,326	12,100
Liabilities:
Contingent purchase price	18,822	19,650
Total liabilities at fair value	18,822	19,650
Fair Value, Measurements, Recurring | Quoted Prices In Active Markets for Identical Assets (Level 1)
Assets:
Cash equivalents	12,237	12,100
Short-term investment	13,089	0
Total assets at fair value	25,326	12,100
Liabilities:
Contingent purchase price	0	0
Total liabilities at fair value	0	0
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2)
Assets:
Cash equivalents	0	0
Short-term investment	0	0
Total assets at fair value	0	0
Liabilities:
Contingent purchase price	0	0
Total liabilities at fair value	0	0
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3)
Assets:
Cash equivalents	0	0
Short-term investment	0	0
Total assets at fair value	0	0
Liabilities:
Contingent purchase price	18,822	19,650
Total liabilities at fair value	$ 18,822	$ 19,650