Revenue Recognition - Dual Reporting - Consolidated Statement of Cash Flows (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net income	$ 8.1	$ 261.4	$ 346.0	$ 80.3	$ 345.9	$ 198.6	$ 227.4	$ (236.4)	$ 695.8	$ 535.5	$ 825.7
Deferred income taxes									(29.0)
Receivables									(10.4)	(91.7)	(53.0)
Contract liabilities									12.1	12.9	21.3
Income taxes									(18.8)	426.7	(22.2)
Other assets and liabilities									(1.8)	$ 14.6	$ (14.8)
Impact of Adoption | Accounting Standards Update 2014-09
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net income									0.8
Deferred income taxes									0.1
Receivables									(1.5)
Contract liabilities									(2.6)
Income taxes									(2.1)
Other assets and liabilities									5.3
Balances without adoption of ASC 606
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net income									696.6
Deferred income taxes									(28.9)
Receivables									(11.9)
Contract liabilities									9.5
Income taxes									(20.9)
Other assets and liabilities									$ 3.5