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Long-Term Debt (Details) - USD ($)
3 Months Ended 6 Months Ended
Mar. 31, 2017
Jun. 30, 2016
Jun. 30, 2017
Uncommitted Credit Facility [Member]      
Line of Credit Facility [Line Items]      
Borrowings Outstanding     $ 0
Commercial Paper [Member]      
Line of Credit Facility [Line Items]      
Average Outstanding Amount   $ 210,000,000  
Weighted average interest rate (in hundredths)   0.76%  
Borrowings Outstanding     0
Revolving Credit Agreement 2020 [Member]      
Line of Credit Facility [Line Items]      
Letter of Credit Facility Current Maximum Capacity     2,000,000,000
Line of Credit Facility, Expiration Date Jul. 21, 2020    
Borrowings Outstanding     $ 0
Eurodollar [Member]      
Line of Credit Facility [Line Items]      
Debt Instrument, Interest Rate, Effective Percentage     2.22%
Base Rate [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%