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Weighted Average Assumptions Used to Determine Net Periodic Cost (Benefit) (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Pension Benefit Plans      
Defined Benefit Plan Disclosure [Line Items]      
Defined Benefit Plan, Future Amortization of Gain (Loss) $ 10.3    
Discount rate 4.45% 4.70% 4.25%
Expected return on plan assets 6.75% 6.75% 7.00%
Other Postretirement Employee Benefit Plans      
Defined Benefit Plan Disclosure [Line Items]      
Defined Benefit Plan, Future Amortization of Gain (Loss) $ 0.9    
Discount rate 4.30% 4.50% 4.15%
Expected return on plan assets 0.00% 0.00% 0.00%