Commitments and Contingencies - Other Contingencies (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2014
Commitments and contingencies
Data security incident expenses	$ 15.6	$ 2.9
Data security incidents
Commitments and contingencies
Insurance recovery		0.6
Selling, general and administrative expenses | 2015 data security incident
Commitments and contingencies
Accrued liability related to loss contingency	12.8
Selling, general and administrative expenses | 2014 data security incident
Commitments and contingencies
Accrued liability related to loss contingency		2.9
Selling, general and administrative expenses | Data security incidents
Commitments and contingencies
Data security incident expenses	$ 14.6	5.6	$ 2.5
Insurance recovery		$ 0.6