Fair Value Measurements - Textual (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Stock Issued During Period, Value, New Issues	$ 27,852,000
Stock Issued During Period, Shares, New Issues		0	0
Fixed Payment [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Business Combination, Contingent Consideration, Liability	3,340,000	$ 4,810,000
Contingent and fixed payments [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Business Combination, Contingent Consideration, Liability	122,426,000	229,564,000
Common Stock [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Stock Issued During Period, Value, New Issues	$ 28,727,000
Stock Issued During Period, Shares, New Issues	3,353,939
Common Class A | Common Stock [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Stock Issued During Period, Value, New Issues	$ 28,727	$ 10,458,000	$ 0
Stock Issued During Period, Shares, New Issues	3,353,939	691,559