Fair Value Measurements - Assets and Liabilities Measured on a Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Liabilities:
Contingent purchase price	$ 82,289	$ 96,957
Fair Value, Measurements, Recurring [Member]
Assets:
Money market	56,097	6,030
Total assets at fair value	56,097	6,030
Liabilities:
Contingent purchase price	137,289	123,757
Total liabilities at fair value	137,289	123,757
Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
Assets:
Money market	56,097	6,030
Total assets at fair value	56,097	6,030
Liabilities:
Contingent purchase price	0	0
Total liabilities at fair value	0	0
Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Assets:
Money market	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 [Member] | Level 3 [Member]
Assets:
Money market	0	0
Total assets at fair value	0	0
Liabilities:
Contingent purchase price	137,289	123,757
Total liabilities at fair value	$ 137,289	$ 123,757