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Debt - Narrative (Details)
3 Months Ended
Jun. 30, 2017
USD ($)
Mar. 31, 2019
USD ($)
Dec. 31, 2018
USD ($)
Debt Instrument [Line Items]      
Long-term debt, fair value   $ 4,000,000,000 $ 3,800,000,000
Five-Year Credit Agreement | Maximum      
Debt Instrument [Line Items]      
Indebtedness to cash flow (not greater than)   4  
Five-Year Credit Agreement | Revolving Credit Facility      
Debt Instrument [Line Items]      
Maximum borrowing capacity $ 1,200,000,000.0    
Credit facility, term (in years) 5 years    
Commitment fee   0.10%  
Five-Year Credit Agreement | Revolving Credit Facility | Minimum      
Debt Instrument [Line Items]      
Commitment fee   0.08%  
Five-Year Credit Agreement | Revolving Credit Facility | Maximum      
Debt Instrument [Line Items]      
Commitment fee   0.175%  
Five-Year Credit Agreement | Commercial paper      
Debt Instrument [Line Items]      
Maximum borrowing capacity   $ 1,200,000,000.0  
Short-term debt   $ 0 $ 0