Fair Value Measurements - Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Liabilities:
Contingent purchase price	$ 34,183	$ 82,289
Fair Value, Measurements, Recurring
Assets:
Cash equivalents	14,686	56,097
Available for sale securities	42,168	0
Total assets at fair value	56,854	56,097
Liabilities:
Contingent purchase price	62,884	137,289
Total liabilities at fair value	62,884	137,289
Fair Value, Measurements, Recurring | Quoted Prices In Active Markets for Identical Assets (Level 1)
Assets:
Cash equivalents	14,686	56,097
Available for sale securities	0	0
Total assets at fair value	14,686	56,097
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
Available for sale securities	42,168	0
Total assets at fair value	42,168	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
Available for sale securities	0	0
Total assets at fair value	0	0
Liabilities:
Contingent purchase price	62,884	137,289
Total liabilities at fair value	$ 62,884	$ 137,289