Recent Accounting Standards - Accounting impact (Details) - New Accounting Pronouncement, Early Adoption, Effect $ in Millions	3 Months Ended	6 Months Ended
	Mar. 31, 2018 USD ($) shares	Mar. 31, 2018 USD ($) shares
New Accounting Pronouncement, Early Adoption [Line Items]
Decrease to the opening balance of retained earnings, Net of Tax	$ 6.1	$ 6.1
Increase to the opening balance of retained earnings, net	19.8	19.8
Decrease in income tax provision due to adoption of ASU No. 2016-9	$ 0.3	$ 12.8
Increase to weighted average diluted shares outstanding | shares	74,891	99,577