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Earnings per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended									12 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Earnings Per Share, Diluted
Net Earnings Available to Common Shareholders	$ 57.0	$ 36.0	$ (191.4)	$ 99.2	$ (61.1)	$ 16.2	$ 43.0	$ 123.6		$ 0.8	$ 121.7	$ 457.3
Dilutive impact of BellRing net earnings										0.0	0.0	0.0
Dilutive preferred stock dividends										0.0	3.0	10.0
Net Income (Loss) Available to Common Stockholders, Diluted										$ 0.8	$ 124.7	$ 467.3
Weighted-average shares for basic earnings per share										68.9	70.8	66.6
Effect of dilutive securities										1.2	4.3	9.3
Weighted-average shares for diluted earnings per share										70.1	75.1	75.9
Basic earnings per common share (in usd per share)	$ 0.85	$ 0.53	$ (2.76)	$ 1.40	$ (0.84)	$ 0.22	$ 0.61	$ 1.85		$ 0.01	$ 1.72	$ 6.87
Diluted earnings per common share (in usd per share)	$ 0.83	$ 0.52	$ (2.76)	$ 1.38	$ (0.84)	$ 0.21	$ 0.58	$ 1.67		$ 0.01	$ 1.66	$ 6.16
Stock Options
Earnings Per Share, Diluted
Effect of dilutive securities										0.6	1.6	1.8
Stock Appreciation Rights
Earnings Per Share, Diluted
Effect of dilutive securities										0.1	0.1	0.1
Restricted Stock
Earnings Per Share, Diluted
Effect of dilutive securities										0.4	0.5	0.4
Performance-based Restricted Stock Units
Earnings Per Share, Diluted
Effect of dilutive securities										0.1	0.0	0.0
Preferred Stock
Earnings Per Share, Diluted
Effect of dilutive securities										0.0	2.1	7.0
Series C Preferred Stock
Earnings Per Share, Diluted
Preferred Stock, Dividend Rate, Percentage							2.50%
Conversion of Stock, Shares Converted							3.2
Series B Preferred Stock
Earnings Per Share, Diluted
Preferred Stock, Dividend Rate, Percentage									3.75%
Conversion of Stock, Shares Converted									1.5
Common Stock
Earnings Per Share, Diluted
Preferred Stock Conversion, Shares Issued							5.9		3.1		5.9	3.1