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Savings, Pension and Other Postretirement Employee Benefit Plans Multiemployer Defined Benefit Plans (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]      
Multiemployer Plans, Withdrawal Obligation $ 78,000    
Entity Tax Identification Number 516031295    
Contributions (in thousands) $ 6,148 $ 6,014 $ 5,960
IAM      
Defined Benefit Plan Disclosure [Line Items]      
Multiemployer Plan Number 002    
Multiemployer Plans, Certified Zone Status Green Green  
Contributions (in thousands) $ 333 $ 335 329
Multiemployer Plans, Surcharge No    
Multiemployer Plans, Collective-Bargaining Arrangement, Expiration Date May 31, 2018    
USW      
Defined Benefit Plan Disclosure [Line Items]      
Entity Tax Identification Number 116166763    
Multiemployer Plan Number 001    
Multiemployer Plans, Certified Zone Status [1] Red Red  
Contributions (in thousands) $ 5,815 $ 5,679 $ 5,631
Multiemployer Plans, Surcharge No    
Multiemployer Plans, Collective-Bargaining Arrangement, Expiration Date Aug. 31, 2017    
Other Postretirement Benefit Plan [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Defined Benefit Plan, Health Care Cost Trend Rate Assumed for Next Fiscal Year 7.40%    
Defined Benefit Plan, Ultimate Health Care Cost Trend Rate 6.70%    
Maximum [Member] | Other Postretirement Benefit Plan [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Defined Benefit Plan, Ultimate Health Care Cost Trend Rate 4.00%    
[1] PIUMPF has been certified as in "Critical and Declining Status" for 2017 and 2016, under the provisions of the Multiemployer Pension Plan Reform Act of 2014.