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Employee plans - Effect of One-Percentage-Point Change in Assumed Health Care Cost Trend Rates (Details)
$ in Millions
12 Months Ended
Dec. 31, 2017
USD ($)
1-Percentage- Point Increase  
Effect on service and interest cost $ 0.1
Effect on postretirement benefit obligation 2.0
1-Percentage- Point Decrease  
Effect on service and interest cost (0.1)
Effect on postretirement benefit obligation $ (1.7)