Fair Value Measurements - Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
	Mar. 31, 2018	Mar. 31, 2018	Sep. 30, 2017	Mar. 31, 2017
2019 Convertible Notes Hedges — Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Expected volatility rate		30.00%
Derivative fair value	$ 11,000	$ 11,000
2019 Convertible Notes Embedded Derivative — Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Expected volatility rate		45.00%
Derivative fair value	$ 21,300	$ 21,300
Recurring | Convertible Hedges and Embedded Derivative Financial Instruments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Expected volatility rate	37.00%
Other assets, net | Recurring | Level 2 | 2019 Convertible Notes Hedges — Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	$ 16,042	16,042	$ 6,591	$ 15,138
Long-term debt, net | Recurring | Level 2 | 2019 Convertible Notes Embedded Derivative — Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liability	$ (16,042)	$ (16,042)	$ (6,591)	$ (15,138)