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Supplemental cash flow information (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016
Condensed Cash Flow Statements, Captions [Line Items]
Interest	$ 7.5	$ 5.8
Income taxes, net	20.5	17.9
Capital Leases	9.5	14.8
Restricted cash	159.4		$ 112.6
Cash and cash equivalents	562.2		614.3
Cash, Cash Equivalents, and Restricted Cash	721.6	$ 288.9	726.9	$ 239.0
Cash from Short Term Borrowings
Condensed Cash Flow Statements, Captions [Line Items]
Restricted cash	28.6		27.0
Cash Held From Customers
Condensed Cash Flow Statements, Captions [Line Items]
Restricted cash	118.6		74.7
Deposits liability
Condensed Cash Flow Statements, Captions [Line Items]
Restricted cash	$ 12.2		$ 10.9