Fair Value Measurements - Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Liabilities:
Contingent purchase price	$ 44,471	$ 82,289
Fair Value, Measurements, Recurring
Assets:
Cash equivalents	106,085	56,097
Available for sale securities	131,545	0
Total assets at fair value	237,630	56,097
Liabilities:
Contingent purchase price	111,171	137,289
Total liabilities at fair value	111,171	137,289
Fair Value, Measurements, Recurring | Quoted Prices In Active Markets for Identical Assets (Level 1)
Assets:
Cash equivalents	12,877	56,097
Available for sale securities	0	0
Total assets at fair value	12,877	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	93,208	0
Available for sale securities	131,545	0
Total assets at fair value	224,753	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	111,171	137,289
Total liabilities at fair value	111,171	137,289
Money market funds
Liabilities:
Cash equivalents	12,900	$ 56,100
Money market funds | Fair Value, Measurements, Recurring | Quoted Prices In Active Markets for Identical Assets (Level 1)
Liabilities:
Cash equivalents	12,900
Corporate debt securities
Liabilities:
Cash equivalents	85,700
Corporate debt securities | Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2)
Liabilities:
Cash equivalents	85,700
U.S. government agency notes
Liabilities:
Cash equivalents	7,500
U.S. government agency notes | Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2)
Liabilities:
Cash equivalents	$ 7,500