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Pension Plans and Other Postretirement Benefits (Details-4)
$ in Thousands
12 Months Ended
Dec. 31, 2017
USD ($)
concentration
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
Other postretirement benefits [Abstract]      
Fair value of plan assets $ 410,121 $ 355,110  
Pension Benefits [Member]      
Defined Benefit Plan, Information about Plan Assets [Abstract]      
Target Plan Asset Allocation 100.00%    
Actual Plan Asset Allocation 100.00% 100.00%  
Other postretirement benefits [Abstract]      
Fair value of plan assets $ 361,651 $ 311,933 $ 290,716
Pension Benefits [Member] | Plan Assets [Member]      
Concentration of Credit Risk [Abstract]      
Concentration risk, percentage 10.00%    
Concentration risk, number of significant concentrations | concentration 0    
Pension Benefits [Member] | Equity Securities [Member]      
Defined Benefit Plan, Information about Plan Assets [Abstract]      
Target Plan Asset Allocation [1] 63.00%    
Actual Plan Asset Allocation 65.00% 65.00%  
Pension Benefits [Member] | United States Equity Securities [Member]      
Defined Benefit Plan, Information about Plan Assets [Abstract]      
Target Plan Asset Allocation 45.50%    
Pension Benefits [Member] | Non-United States Equity Securities [Member]      
Defined Benefit Plan, Information about Plan Assets [Abstract]      
Target Plan Asset Allocation 17.50%    
Pension Benefits [Member] | Fixed Income Securities [Member]      
Defined Benefit Plan, Information about Plan Assets [Abstract]      
Target Plan Asset Allocation 30.00%    
Actual Plan Asset Allocation 29.00% 27.00%  
Pension Benefits [Member] | Real Estate [Member]      
Defined Benefit Plan, Information about Plan Assets [Abstract]      
Target Plan Asset Allocation 7.00%    
Actual Plan Asset Allocation 6.00% 8.00%  
Postretirement Benefits [Member]      
Other postretirement benefits [Abstract]      
Fair value of plan assets $ 48,470 $ 43,177 $ 40,170
Postretirement Benefits [Member] | Plan Assets [Member]      
Concentration of Credit Risk [Abstract]      
Concentration risk, percentage 10.00%    
Concentration risk, number of significant concentrations | concentration 0    
Master Pension Trust [Member]      
Other postretirement benefits [Abstract]      
Fair value of plan assets $ 43,100 $ 37,000  
[1]

Target allocations for equity securities are broken out as follows: 45.5 % United States equity,

17.5 % non- United States equity .