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Debt (Narrative) (Details)
6 Months Ended 12 Months Ended
Jun. 30, 2016
USD ($)
Jun. 30, 2015
USD ($)
Dec. 31, 2015
USD ($)
Debt Instrument [Line Items]      
Long-term debt fair value $ 3,800,000,000   $ 3,600,000,000
Five-Year Credit Agreement | Maximum      
Debt Instrument [Line Items]      
Indebtedness to cash flow 4    
Five-Year Credit Agreement | Revolving Credit Facility      
Debt Instrument [Line Items]      
Maximum borrowing capacity   $ 1,200,000,000.0  
Commitment fee 0.15%    
Five-Year Credit Agreement | Revolving Credit Facility | Minimum      
Debt Instrument [Line Items]      
Commitment fee 0.10%    
Five-Year Credit Agreement | Revolving Credit Facility | Maximum      
Debt Instrument [Line Items]      
Commitment fee 0.20%    
Five-Year Credit Agreement | Commercial paper      
Debt Instrument [Line Items]      
Short-term debt $ 309,000,000   $ 143,000,000
Five-Year Credit Agreement | Commercial paper | Revolving Credit Facility      
Debt Instrument [Line Items]      
Average interest rate 0.89%   0.95%
Term of debt instrument 14 days   17 days
Line of Credit | Five-Year Credit Agreement | Revolving Credit Facility      
Debt Instrument [Line Items]      
Term of debt instrument   5 years  
Additional funds remaining in credit facility $ 891,000,000