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Recent accounting pronouncements and accounting changes - Additional Information (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
New Accounting Pronouncements or Change in Accounting Principle [Line Items]      
Cumulative-effect adjustment $ 4.4    
Reclassification from operating activities, Other assets 27.5 $ (16.5) [1] $ (18.1) [1]
Reclassification to investing activities, Other (0.8) (0.9) [1] 7.0 [1]
Reclassification from operating activities, Accounts payable and other accrued liabilities 48.6 7.5 [1] (60.3) [1]
Reclassification to financing activities, Other 2.3 0.4 [1] 0.8 [1]
Accounting Standards Update 2016-15      
New Accounting Pronouncements or Change in Accounting Principle [Line Items]      
Reclassification from operating activities, Other assets   1.5 1.1
Reclassification to investing activities, Other   1.5 1.1
Accounting Standards Update 2016-09      
New Accounting Pronouncements or Change in Accounting Principle [Line Items]      
Reclassification from operating activities, Accounts payable and other accrued liabilities   0.4 0.8
Reclassification to financing activities, Other   0.4 $ 0.8
Accounting Standards Update 2017-07      
New Accounting Pronouncements or Change in Accounting Principle [Line Items]      
Net periodic benefit cost, other than service cost 92.5 81.6  
Accumulated Deficit      
New Accounting Pronouncements or Change in Accounting Principle [Line Items]      
Cumulative-effect adjustment $ 4.4    
Accumulated Deficit | Accounting Standards Update 2016-16      
New Accounting Pronouncements or Change in Accounting Principle [Line Items]      
Cumulative-effect adjustment   $ 4.4  
[1] Amounts were changed to conform to the current-year presentation. See Note 5