Long-Term Debt (Details) - USD ($)	3 Months Ended	6 Months Ended
	Jun. 30, 2015	Jun. 30, 2015	Jun. 30, 2014
Debt Instrument Issuance [Abstract]
Net proceeds from Issuance of Senior Long-Term Debt	$ 990,000,000
Revolving Credit Agreement [Member]
Line of Credit Facility [Line Items]
Letters of Credit Outstanding	0	$ 0
Borrowings Outstanding	$ 0	0
Line of Credit Facility, Expiration Date	Oct. 11, 2016
Letter of Credit Facility Previous Maximum Capacity	$ 2,000,000,000	$ 2,000,000,000
Eurodollar rate at period end (in hundredths)	1.062%	1.062%
Base rate at period end (in hundredths)	3.25%	3.25%
Uncommitted Credit Facility [Member]
Line of Credit Facility [Line Items]
Borrowings Outstanding	$ 0	$ 0
Average Outstanding Amount			$ 200,000
Weighted average interest rate (in hundredths)			0.70%
Commercial Paper [Member]
Line of Credit Facility [Line Items]
Average Outstanding Amount			$ 18,000,000
Weighted average interest rate (in hundredths)			0.25%
New Facility [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Expiration Date		Jul. 21, 2020
Advances up to an aggregate principal amount		$ 2,000,000,000
Letter of Credit Facility Current Maximum Capacity	$ 2,000,000,000	2,000,000,000
Increases in aggregate commitments to amount not to exceed		$ 3,000,000,000
Maximum total debt to capitalization ratio allowed under financial convenant	65.00%	65.00%
3.90% Senior Notes due 2035 [Member]
Debt Instrument Issuance [Abstract]
Debt Instrument Issuance Face Amount	$ 500,000,000	$ 500,000,000
Debt Instrument Issuance Interest Rate	3.90%	3.90%
3.15 % Senior Notes due 2025 [Member]
Debt Instrument Issuance [Abstract]
Debt Instrument Issuance Face Amount	$ 500,000,000	$ 500,000,000
Debt Instrument Issuance Interest Rate	3.15%	3.15%
2.500% Senior Notes Due 2016 [Member]
Debt Instrument Issuance [Abstract]
Debt Instrument Issuance Face Amount	$ 400,000,000	$ 400,000,000
Debt Instrument Issuance Interest Rate	2.50%	2.50%
2.95% Senior Notes Due 2015 [Member]
Debt Instrument Issuance [Abstract]
Debt Instrument Issuance Face Amount	$ 500,000,000	$ 500,000,000
Debt Instrument Issuance Interest Rate	2.95%	2.95%