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Debt - Lines of Credit and Other Debt Facilities (Details) - USD ($)
9 Months Ended 12 Months Ended
Sep. 30, 2018
Dec. 31, 2017
Debt Instrument [Line Items]    
Debt and capital lease obligations $ 5,399,600,000 $ 5,447,200,000
Line of Credit    
Debt Instrument [Line Items]    
Outstanding letters of credit 17,500,000 19,300,000
Reduction in borrowings 16,800,000 18,600,000
Line of Credit | Lines of Credit and Revolving Lines of Credit    
Debt Instrument [Line Items]    
Debt and capital lease obligations 0 0
Remaining borrowing capacity $ 497,000,000 $ 509,000,000