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Stockholders' equity - Amounts Reclassified Out of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Accumulated Other Comprehensive Income (Loss) [Line Items]      
Other income (expense), net $ (23.9) $ 0.3 $ 8.2
Total reclassifications for the period (169.8) (142.6) (178.1)
Translation Adjustments      
Accumulated Other Comprehensive Income (Loss) [Line Items]      
Total reclassifications for the period (11.8) 0.0 0.0
Amortization of prior service cost      
Accumulated Other Comprehensive Income (Loss) [Line Items]      
Amortization of postretirement plan items, before tax 5.6 5.6 3.1
Amortization of actuarial losses      
Accumulated Other Comprehensive Income (Loss) [Line Items]      
Amortization of postretirement plan items, before tax (174.1) (155.2) (189.7)
Curtailment gain      
Accumulated Other Comprehensive Income (Loss) [Line Items]      
Amortization of postretirement plan items, before tax 5.4 2.0 0.0
Postretirement Plans      
Accumulated Other Comprehensive Income (Loss) [Line Items]      
Amortization of postretirement plan items, before tax (174.9) (147.6) (186.6)
Income tax benefit 5.1 5.0 8.5
Total reclassifications for the period (158.0) (142.6) (178.1)
Adjustment for substantial completion of liquidation of foreign subsidiaries | Translation Adjustments      
Accumulated Other Comprehensive Income (Loss) [Line Items]      
Other income (expense), net $ (11.8) $ 0.0 $ 0.0