Debt - Capital Lease Obligations (Details) - USD ($) $ in Millions	Jun. 30, 2018	Sep. 30, 2017
Debt Instrument [Line Items]
Capital lease obligation balance	$ 834.3	$ 823.7
Annual interest payments	16.0
Capital lease obligations
Debt Instrument [Line Items]
Capital lease obligation balance	34.9	$ 37.5
2015 Ryder Lease | Ryder
Debt Instrument [Line Items]
Capital lease, aggregate future interest payments	3.0
2015 Ryder Lease | Ryder | Maximum
Debt Instrument [Line Items]
Annual interest payments	0.9
2015 Ryder Lease | Ryder | Minimum
Debt Instrument [Line Items]
Annual interest payments	0.1
Montgomery Lease
Debt Instrument [Line Items]
Capital lease, aggregate future interest payments	12.9
Montgomery Lease | Maximum
Debt Instrument [Line Items]
Annual interest payments	1.0
Montgomery Lease | Minimum
Debt Instrument [Line Items]
Annual interest payments	$ 0.1