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Employee plans - Assumed Health Care Cost Trend Rates (Details)
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Retirement Benefits [Abstract]    
Health care cost trend rate assumed for next year 6.80% 6.60%
Rate to which the cost trend rate is assumed to decline (the ultimate trend rate) 4.80% 4.80%
Year that the rate reaches the ultimate trend rate 2023 2023