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Long-Term Debt (Credit Facility End of Period Disclosures Narrative) (Details)
12 Months Ended
Dec. 31, 2017
USD ($)
item
Credit Facility, Name [Member] | Credit Facility, Type of Debt [Member]  
Debt Instrument [Line Items]  
Minimum Number of Additional Tranches Available in Future | item 1
Aggregate Principal Amount of Each Tranche Available in Future $ 1,500,000,000
Debt, Weighted Average Interest Rate 5.10%
Credit Facility, Name [Member] | Credit Facility, Type of Debt, Amendment No. 2 [Member]  
Debt Instrument [Line Items]  
Aggregate Principal Amount of Each Tranche Available in Future $ 1,000,000,000
Credit Facility, Revolving Credit Loans [Member] | Line of Credit [Member]  
Debt Instrument [Line Items]  
Line of Credit Facility, Remaining Borrowing Capacity 929,000,000
Letters of Credit Outstanding, Amount $ 63,000,000