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Retirement and Deferred Compensation Plans - Plans with Accumulated Benefit Obligations in Excess of Fair Value of Plan Assets (Details) - USD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Compensation and Retirement Disclosure [Abstract]    
Accumulated benefit obligation $ 135.9 $ 107.9
Plan assets $ 65.5 $ 47.9