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Employee plans - Components of Net Periodic Benefit Expense (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Pension Plans      
Defined Benefit Plan Disclosure [Line Items]      
Amortization of prior service credit $ (4.9)    
Recognized net actuarial loss 176.2    
Pension Plans | U.S. Plans      
Defined Benefit Plan Disclosure [Line Items]      
Service cost 0.0 $ 0.0 $ 0.0
Interest cost 211.3 231.3 224.1
Expected return on plan assets (235.2) (253.1) (254.8)
Amortization of prior service credit (2.5) (2.5) (2.4)
Recognized net actuarial loss 126.4 116.0 132.7
Curtailment gain 0.0 0.0 0.0
Net periodic pension cost (income) 100.0 91.7 99.6
Pension Plans | International Plans      
Defined Benefit Plan Disclosure [Line Items]      
Service cost 5.1 7.4 8.7
Interest cost 72.8 87.8 94.1
Expected return on plan assets (127.5) (139.5) (155.4)
Amortization of prior service credit (2.4) (3.0) (1.9)
Recognized net actuarial loss 49.8 40.3 63.6
Curtailment gain (5.4) (2.0) 0.0
Net periodic pension cost (income) (7.6) (9.0) 9.1
Other Postretirement Benefit Plans      
Defined Benefit Plan Disclosure [Line Items]      
Service cost 0.5 0.4 0.6
Interest cost 5.6 6.2 6.9
Expected return on plan assets (0.5) (0.4) (0.4)
Amortization of prior service credit (0.7) 0.0 1.1
Recognized net actuarial loss 0.8 0.5 1.8
Net periodic pension cost (income) $ 5.7 $ 6.7 $ 10.0