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Employee Retirement Benefits - Narrative (Details) - USD ($)
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Provision for defined contribution plans $ 9,400,000 $ 7,500,000 $ 7,400,000
Offset to taxes (2,000,000) (1,996,300,000) $ 1,003,000,000
Effect of 1% increase in service and interest cost (less than) 100,000    
Effect of 1% increase in postretirement benefit obligation (less than) 100,000    
Qualified Pension Plan      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Increase to the pension liability funded status (3,600,000) 3,200,000  
Company contributions 0 $ 100,000  
Net actuarial loss $ 600,000    
Expected return on assets 6.50% 7.20% 7.40%
Equity Securities      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Asset allocation percentage 35.00%    
Fixed Income Securities      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Asset allocation percentage 65.00%    
Domestic Plan      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Defined Benefit Plan, Actual Benefit Plan Obligation Allocations 98.00%    
Company contributions $ 0 $ 0 $ 0
Asset allocation percentage 100.00%    
Additional Paid-In Capital      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Offset to taxes $ (2,000,000) $ (1,996,300,000) $ 1,003,000,000
Additional Paid-In Capital | Domestic Plan      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Offset to taxes $ (2,000,000)