Income Taxes - Reconciliation to effective income tax rate (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2018	Mar. 31, 2017
Components Of Deferred Income Tax Assets And Liabilities [Line Items]
Reported income tax expense	$ (1.2)	$ 24.0	$ (127.2)	$ 29.6
Effective Income Tax Rate Reconciliation, Change in Deferred Tax Assets Valuation Allowance, Amount	$ 10.4
Effective Income Tax Rate Reconciliation, Percent	3.10%	148.10%	147.20%	(146.50%)