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Long-Term Debt (Details) - USD ($)
9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Commercial Paper [Member]    
Line of Credit Facility [Line Items]    
Borrowings Outstanding $ 0 $ 0
Average Outstanding Amount $ 9,000,000 $ 155,000,000
Weighted average interest rate (in hundredths) 1.39% 0.76%
Uncommitted Credit Facility [Member]    
Line of Credit Facility [Line Items]    
Borrowings Outstanding $ 0 $ 0
Revolving Credit Agreement 2020 [Member]    
Line of Credit Facility [Line Items]    
Borrowings Outstanding 0  
Letter of Credit Facility Current Maximum Capacity $ 2,000,000,000  
Line of Credit Facility, Expiration Date Jul. 21, 2020  
Eurodollar [Member] | Revolving Credit Agreement 2020 [Member]    
Line of Credit Facility [Line Items]    
Debt Instrument, Interest Rate, Effective Percentage 2.23%  
Base Rate [Member] | Revolving Credit Agreement 2020 [Member]    
Line of Credit Facility [Line Items]    
Debt Instrument, Interest Rate, Effective Percentage 4.25%  
5.875 % Senior Notes Due 2017 [Member]    
Debt Instrument [Abstract]    
Debt Instrument Issuance Face Amount $ 600,000,000  
Debt Instrument Issuance Interest Rate 5.875%