EARNINGS PER COMMON SHARE (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Basic Earnings Per Share:
Net Income	$ 25.9	$ 21.7	$ 240.1	$ 174.9
Less: Income allocated to participating securities	0.1	0.1	0.5	0.4
Net Income Available to Common Shareholders	$ 25.8	$ 21.6	$ 239.6	$ 174.5
Weighted Average Shares Outstanding (in shares)	49.6	48.1	48.7	46.4
Basic Earnings Per Share of Common Stock (in dollars per share)	$ 0.52	$ 0.45	$ 4.92	$ 3.76
Diluted Earnings Per Share:
Net Income	$ 25.9	$ 21.7	$ 240.1	$ 174.9
Less: Income allocated to participating securities	0.1	0.1	0.5	0.4
Net Income Available to Common Shareholders	$ 25.8	$ 21.6	$ 239.6	$ 174.5
Weighted Average Shares Outstanding (in shares)	49.6	48.1	48.7	46.4
Dilutive Effect of Restricted Stock and Restricted Stock Units (in shares)	0.1	0.1	0.1	0.2
Weighted Average Diluted Shares (in shares)	49.7	48.2	48.8	46.6
Diluted Earnings Per Share of Common Stock (in dollars per share)	$ 0.52	$ 0.45	$ 4.91	$ 3.75
Restricted stock and stock units subject to performance and/or market conditions
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Outstanding shares excluded from calculation of diluted EPS (in shares)	0.4	0.4	0.4	0.4