Earnings per Share - Schedule of Earnings per Share (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Basic:
Net income	$ 17.5	$ 10.2	$ 44.4	$ 0.8
Weighted average number of common shares outstanding during the period (in shares)	76,797,414	76,743,853	76,795,551	76,745,396
Net income per common share - basic (USD per share)	$ 0.23	$ 0.13	$ 0.58	$ 0.01
Diluted:
Net income	$ 17.5	$ 10.2	$ 44.4	$ 0.8
Denominator for diluted earnings per share:
Weighted average number of common shares outstanding during the period (in shares)	76,797,414	76,743,853	76,795,551	76,745,396
Incremental common shares attributable to outstanding unvested restricted stock and unvested restricted stock units(in shares)	185,936	85,015	157,207	84,900
Denominator for diluted earnings per common share (in shares)	76,983,350	76,828,868	76,952,758	76,830,296
Net income per common share - diluted (USD per share)	$ 0.23	$ 0.13	$ 0.58	$ 0.01