Income Taxes (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Sep. 30, 2018
Income Tax Disclosure [Abstract]
Income tax expense	$ 89.0	$ 202.4
Effective tax rate (percent)	23.70%	51.70%
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent		35.00%	24.50%
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Percent	21.00%
Deferred tax assets net of DTL	$ 198.5		$ 211.7
Valuation allowance for deferred income taxes	$ 17.1		$ 17.7