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Employee Benefit Plans - Investment of Plan Assets (Details) - Pension
Dec. 31, 2017
Dec. 31, 2016
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Percentage of Plan Assets 100.00% 100.00%
Domestic equity securities    
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Percentage of Plan Assets 41.00% 41.00%
International equity securities    
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Percentage of Plan Assets 26.00% 25.00%
Domestic fixed income securities    
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Percentage of Plan Assets 26.00% 26.00%
International fixed income securities    
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Percentage of Plan Assets 4.00% 5.00%
Real estate fund    
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Percentage of Plan Assets 3.00% 3.00%
Minimum | Domestic equity securities    
Defined Benefit Plan, Assets, Target Allocations [Abstract]    
Target allocation range minimum, percent 35.00%  
Minimum | International equity securities    
Defined Benefit Plan, Assets, Target Allocations [Abstract]    
Target allocation range minimum, percent 20.00%  
Minimum | Domestic fixed income securities    
Defined Benefit Plan, Assets, Target Allocations [Abstract]    
Target allocation range minimum, percent 25.00%  
Minimum | International fixed income securities    
Defined Benefit Plan, Assets, Target Allocations [Abstract]    
Target allocation range minimum, percent 3.00%  
Minimum | Real estate fund    
Defined Benefit Plan, Assets, Target Allocations [Abstract]    
Target allocation range minimum, percent 2.00%  
Maximum | Domestic equity securities    
Defined Benefit Plan, Assets, Target Allocations [Abstract]    
Target allocation range minimum, percent 45.00%  
Maximum | International equity securities    
Defined Benefit Plan, Assets, Target Allocations [Abstract]    
Target allocation range minimum, percent 30.00%  
Maximum | Domestic fixed income securities    
Defined Benefit Plan, Assets, Target Allocations [Abstract]    
Target allocation range minimum, percent 29.00%  
Maximum | International fixed income securities    
Defined Benefit Plan, Assets, Target Allocations [Abstract]    
Target allocation range minimum, percent 7.00%  
Maximum | Real estate fund    
Defined Benefit Plan, Assets, Target Allocations [Abstract]    
Target allocation range minimum, percent 4.00%