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Retirement Benefit Plans - Health Care Trend Rates (Details)
$ in Millions
12 Months Ended
Dec. 31, 2018
USD ($)
Assumed health care cost trend rates [Abstract]  
Health care cost trend rate assumed for next fiscal year 6.50%
Ultimate health care cost trend rate 5.00%
Effect of one-percentage point change in assumed health care cost trend rates [Abstract]  
Effect of one percentage point increase on accumulated postretirement benefit obligation $ 5.5
Effect of one percentage point decrease on accumulated postretirement benefit obligation (4.9)
Effect of one percentage point increase on service and interest cost components 0.2
Effect of one percentage point decrease on service and interest cost components $ (0.2)