Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2021	Mar. 31, 2020
Statement Of Income And Comprehensive Income [Abstract]
Net income (loss)	$ (6.3)	$ (122.6)	$ (226.8)	$ (146.0)
Other comprehensive income (loss):
Foreign currency translation adjustment	1.0	(6.4)	4.9	(5.1)
Unrealized gain (loss) due to change in fair value of derivatives, net of tax	3.4	(15.9)	5.0	(13.4)
Total other comprehensive income (loss)	4.4	(22.3)	9.9	(18.5)
Comprehensive income (loss)	$ (1.9)	$ (144.9)	$ (216.9)	$ (164.5)

Amount after tax of increase (decrease) in equity from transactions and other events and circumstances from net income and other comprehensive income, attributable to parent entity. Excludes changes in equity resulting from investments by owners and distributions to owners.

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Amount after tax, before reclassification adjustments of gain (loss) on foreign currency translation adjustments, foreign currency transactions designated and effective as economic hedges of a net investment in a foreign entity and intra-entity foreign currency transactions that are of a long-term-investment nature.

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Amount, after tax and before reclassification, of gain (loss) from derivative instrument designated and qualifying as cash flow hedge included in assessment of hedge effectiveness.

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