EARNINGS PER COMMON SHARE (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions		3 Months Ended								12 Months Ended
	Apr. 03, 2017	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
Basic Earnings Per Share:
Net income		$ (25.9)	$ 25.9	$ 98.2	$ 116.0	$ (13.3)	$ 21.7	$ 108.0	$ 45.2	$ 214.2	$ 161.6	$ 144.2
Less: Income allocated to participating securities										0.5	0.4	0.5
Net Income Available to Common Shareholders										$ 213.7	$ 161.2	$ 143.7
Weighted Average Shares Outstanding, Basic (in shares)										49,100	46,900	44,100
Earnings Per Share of Common Stock (in dollars per share)		$ (0.51)	$ 0.52	$ 2.03	$ 2.40	$ (0.28)	$ 0.45	$ 2.36	$ 0.99	$ 4.35	$ 3.44	$ 3.26
Diluted Earnings per Share:
Net income		$ (25.9)	$ 25.9	$ 98.2	$ 116.0	$ (13.3)	$ 21.7	$ 108.0	$ 45.2	$ 214.2	$ 161.6	$ 144.2
Less: Income allocated to participating securities										0.5	0.4	0.5
Net Income Available to Common Shareholders										$ 213.7	$ 161.2	$ 143.7
Weighted Average Shares Outstanding, Basic (in shares)										49,100	46,900	44,100
Dilutive Effect of Stock Options, Restricted Stock, and Restricted Stock Units (in shares)										200	100	200
Weighted Average Diluted Shares (in shares)										49,300	47,000	44,300
Earnings Per Share of Common Stock (in dollars per share)		$ (0.51)	$ 0.52	$ 2.03	$ 2.39	$ (0.28)	$ 0.45	$ 2.36	$ 0.99	$ 4.33	$ 3.43	$ 3.24
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Outstanding shares excluded from calculation of diluted EPS (in shares)	2,500									2,875
Calculation excludes certain outstanding shares (shown in millions by period at the right) attributable to stock units subject to performance or market conditions and restricted stock, which could have a dilutive effect in the future
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Outstanding shares excluded from calculation of diluted EPS (in shares)										400	500	300