Debt - Components of Long Term Debt (Details) - USD ($) $ in Millions	Jun. 30, 2017	Sep. 30, 2016
Debt Instrument [Line Items]
Total debt	$ 995.7	$ 1,000.0
Less debt discount	2.6	2.8
Less deferred debt costs	9.3	11.1
Less current portion of long-term debt	5.8	5.8
Long-term debt payable after one year	978.0	980.3
Term loan facility under Credit Agreement
Debt Instrument [Line Items]
Total debt	570.7	575.0
Line of Credit | Revolving facility under Credit Agreement
Debt Instrument [Line Items]
Total debt	0.0	0.0
Senior Notes | 5.500% Senior Notes due 2024
Debt Instrument [Line Items]
Total debt	$ 425.0	$ 425.0
Interest rate (as percent)	5.50%	5.50%