Basis of Presentation and Accounting Policies - Reconciliation of Baisc and Diluted Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended								12 Months Ended
	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Reconciled Basic and Diluted EPS
Net income	$ 345.9	$ 198.6	$ 227.4	$ (236.4)	$ 204.6	$ 216.9	$ 189.5	$ 214.7	$ 535.5	$ 825.7	$ 729.7
Less: Allocation to participating securities									(0.5)	(0.9)	(0.7)
Net income available to common shareowners									$ 535.0	$ 824.8	$ 729.0
Basic weighted average outstanding shares (in shares)									125.4	128.4	130.2
Effect of dilutive securities
Stock options (in shares)									1.3	1.2	0.9
Performance shares (in shares)									0.2	0.3	0.0
Diluted weighted average outstanding shares (in shares)									126.9	129.9	131.1
Earnings per share:
Basic (in dollars per share)	$ 2.84	$ 1.60	$ 1.79	$ (1.84)	$ 1.59	$ 1.69	$ 1.47	$ 1.67	$ 4.27	$ 6.42	$ 5.60
Diluted (in dollars per share)	$ 2.80	$ 1.58	$ 1.77	$ (1.84)	$ 1.57	$ 1.67	$ 1.45	$ 1.65	$ 4.21	$ 6.35	$ 5.56