Income taxes - Additional Information (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Tax Cuts and Jobs Act of 2017 provisional income tax expense (benefit)	$ (105.5)		$ (128.2)
Change in tax rate effect on deferred tax liabilities	202.9		210.0
Transition tax	$ 97.4		$ 81.8
Income tax expense at federal statutory rate, rate	21.00%	35.00%
Income Tax Contingency [Line Items]
Adjustment to provisional income tax expense (benefit)	$ 22.7
State and Local Jurisdiction
Income Tax Contingency [Line Items]
Adjustment to provisional income tax expense (benefit)	$ 9.3