Long-Term Debt - Impact of Early Conversions, Convertible Notes Hedges and Warrants (Details) $ / shares in Units, $ in Thousands, shares in Millions	12 Months Ended
	Sep. 30, 2017 USD ($) $ / shares $ / share shares	Sep. 30, 2016 USD ($)
Debt Instrument [Line Items]
Debt discount and issuance costs	$ 60,534	$ 34,081
Convertible Notes Hedges
Debt Instrument [Line Items]
Cost of hedge	46,500
Cost of hedge, net of total proceeds from Warrants sold	$ 21,300
Common Class A | Convertible Notes Hedges
Debt Instrument [Line Items]
Notional amount (shares) | shares	12.1
Strike price (usd per share) | $ / share	16.065
Net-share-settled Warrants | Common Class A
Debt Instrument [Line Items]
Maximum shares called by warrant (shares) | shares	14.3
Strike price for warrants (usd per share) | $ / shares	$ 20.83
Reclassification of 2019 Convertible Notes Warrants to liabilities	$ 25,100
Warrants outstanding (shares) | shares	12.1
2.125% Cash Convertible Senior Notes Due 2019
Debt Instrument [Line Items]
Debt discount and issuance costs	$ 17,654	$ 32,046
2.125% Cash Convertible Senior Notes Due 2019 | If-Converted Scenario
Debt Instrument [Line Items]
Debt discount and issuance costs	$ 17,700