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Retirement and Deferred Compensation Plans - Effect of One-Percentage Point Change in Assumed Health Care Cost Trend Rates (Details) - Retiree health care plan
$ in Millions
12 Months Ended
Dec. 31, 2018
USD ($)
Defined Benefit Plan Disclosure [Line Items]  
Effect of 1% increase on total of service and interest cost components $ 0.0
Effect of 1% decrease on total of service and interest cost components 0.0
Effect of 1% increase on benefit obligation 0.3
Effect of 1% decrease on benefit obligation $ (0.3)