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Weighted Average Assumptions Used to Determine Benefit Obligation (Details)
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Pension Benefit Plans      
Defined Benefit Plan Disclosure [Line Items]      
Discount rate 3.90% 4.45% 4.70%
Other Postretirement Employee Benefit Plans      
Defined Benefit Plan Disclosure [Line Items]      
Discount rate 3.95% 4.30% 4.50%