Income taxes - Additional Information (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Tax Cuts and Jobs Act of 2017 provisional income tax expense (benefit)			$ (128.2)
Change in tax rate effect on deferred tax liabilities			210.0
Transition tax			$ 81.8
Income tax expense at federal statutory rate, rate	21.00%	35.00%