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Long-Term Debt - Credit Facilities (Details) - USD ($)
3 Months Ended 12 Months Ended
Mar. 31, 2018
Dec. 31, 2017
Jul. 26, 2017
Revolving Credit Facility      
Line of Credit Facility [Line Items]      
Letters of credit $ 5,000,000    
Average interest rate 3.61%    
Borrowings against Revolving Credit Facility $ 273,000,000 $ 275,000,000  
Revolving Credit Facility | A&R Credit Agreement      
Line of Credit Facility [Line Items]      
Revolving credit facility maximum borrowing capacity     $ 500,000,000
Term Loan | Term Loan A      
Line of Credit Facility [Line Items]      
Revolving credit facility maximum borrowing capacity     688,000,000
Percentage bearing variable interest, percentage rate 3.51%    
Term Loan | Term Loan B      
Line of Credit Facility [Line Items]      
Revolving credit facility maximum borrowing capacity     $ 2,200,000,000
Percentage bearing variable interest, percentage rate 3.75%    
Term Loan | Term Loan B | Floor      
Line of Credit Facility [Line Items]      
Percentage bearing variable interest, percentage rate 2.75%