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Retirement and Deferred Compensation Plans - Projected Benefit Payment Estimates (Details)
$ in Millions
Dec. 31, 2017
USD ($)
Pension plans  
Defined Benefit Plan Disclosure [Line Items]  
2018 $ 14.0
2019 14.3
2020 15.8
2021 17.9
2022 19.6
2023-2027 127.8
Total projected benefit payments 209.4
Retiree Health Care Plan  
Defined Benefit Plan Disclosure [Line Items]  
2018 1.2
2019 1.2
2020 1.1
2021 1.1
2022 1.1
2023-2027 5.0
Total projected benefit payments $ 10.7