Revenue Recognition - Dual Reporting - Consolidated Statement of Shareowners' Equity (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	Oct. 01, 2018	Sep. 30, 2016
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Beginning balance	$ 404.2				$ 1,617.5				$ 404.2	$ 1,617.5	$ 2,663.6		$ 1,990.1
Net income	8.1	$ 261.4	$ 346.0	$ 80.3	345.9	$ 198.6	$ 227.4	$ (236.4)	695.8	535.5	825.7
Other comprehensive income									(546.1)	237.3	359.6
Impact of Adoption | Accounting Standards Update 2014-09
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net income									0.8
Balances without adoption of ASC 606
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net income									696.6
Retained earnings
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Beginning balance	6,440.2				6,198.1				6,440.2	6,198.1	6,103.4	$ 6,204.2	5,668.4
Net income									695.8	535.5	825.7
Retained earnings | Impact of Adoption | Accounting Standards Update 2014-09
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Beginning balance												(6.1)
Net income									0.8
Retained earnings | Balances without adoption of ASC 606
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Beginning balance												$ 6,198.1
Net income									696.6
Accumulated other comprehensive income/(loss)
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Beginning balance	$ (1,488.0)				$ (941.9)				(1,488.0)	$ (941.9)	$ (1,179.2)		$ (1,538.8)
Other comprehensive income									(1,488.0)
Accumulated other comprehensive income/(loss) | Impact of Adoption | Accounting Standards Update 2014-09
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Other comprehensive income									0.4
Accumulated other comprehensive income/(loss) | Balances without adoption of ASC 606
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Other comprehensive income									$ (1,487.6)