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Pension and Other Postretirement Benefit Plans - Pension Plans with Accumulated Benefit Obligations in Excess of Plan Assets (Details) - USD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Retirement Benefits [Abstract]    
Projected benefit obligation $ 768.1 $ 747.3
Accumulated benefit obligation 754.1 730.4
Fair value of plan assets $ 525.7 $ 496.0