Long-Term Debt - 2019 Convertible Notes Hedges and Warrants (Details) $ / shares in Units, shares in Millions, $ in Millions	12 Months Ended
	Sep. 30, 2018 USD ($) $ / shares $ / share shares
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 | $	$ 25.1
Warrants outstanding (shares) | shares	12.1
Convertible Notes Hedges
Debt Instrument [Line Items]
Cost of hedge | $	$ 46.5
Cost of hedge, net of total proceeds from Warrants sold | $	$ 21.3
Convertible Notes Hedges | Common Class A
Debt Instrument [Line Items]
Notional amount (shares) | shares	12.1
Strike price (usd per share) | $ / share	16.065