Revenue Recognition - Dual Reporting - Consolidated Statement of Cash Flows (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2019	Mar. 31, 2018
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net income (loss)	$ 346.0	$ 227.4	$ 426.3	$ (9.0)
Receivables			(41.5)	(9.8)
Contract liabilities			41.2	42.8
Income taxes			(66.7)	508.6
Other assets and liabilities			(11.8)	$ (4.9)
Impact of Adoption | Accounting Standards Update 2014-09
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net income (loss)	(4.8)		(2.5)
Receivables			(10.7)
Contract liabilities			15.4
Income taxes			(1.4)
Other assets and liabilities			(0.8)
Balances without adoption of ASC 606
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net income (loss)	$ 341.2		423.8
Receivables			(52.2)
Contract liabilities			56.6
Income taxes			(68.1)
Other assets and liabilities			$ (12.6)