Fair Value Measurements - Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - Recurring - USD ($) $ in Thousands	3 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
Convertible Hedges and Embedded Derivative Financial Instruments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Expected volatility rate	36.00%
Other assets, net | Level 2 | 2019 Convertible Notes Hedges — Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	$ 12,863	$ 6,591	$ 29,800
Long-term debt, net | Level 2 | 2019 Convertible Notes Embedded Derivative — Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liability	(12,863)	(6,591)	(29,800)
Accounts payable, accrued expenses and other current liabilities | Deferred and contingent consideration
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liability	$ (1,920)	$ 0	$ 0