Debt - Revolving Credit and Letter of Credit (Details) (USD $)	1 Months Ended	12 Months Ended
	Dec. 31, 2014 Banks	Dec. 31, 2014 Banks
Line of Credit Facility [Line Items]
Short-term and Long-term Debt Outstanding	$ 5,200,000,000	$ 5,200,000,000
Revolving Credit And Letter Of Credit Facility [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Initiation Date	Dec. 01, 2014
Line of Credit Facility, Expiration Date	Dec. 01, 2019
Line of Credit Facility, Term of Agreement	5 years
Line of Credit Facility, Maximum Borrowing Capacity	1,500,000,000	1,500,000,000
Line of Credit Facility, Number of Participating Banks	16	16
Line of Credit Facility, Optional Maximum Borrowing Capacity	2,000,000,000	2,000,000,000
Line of Credit Facility, Covenant Terms		This agreement has certain covenants, including a financial covenant requiring the Company to maintain a leverage ratio of total consolidated debt-to-consolidated capitalization (each defined in the credit agreement) at or below 0.50.
Maximum consolidated debt-to-consolidated capitalization ratio convenant	50.00%	50.00%
Credit Facility Remaining Borrowing Capacity	6,500,000,000	6,500,000,000
Line of Credit Facility, Covenant Compliance		The Company was in compliance with its debt covenants as of December 31, 2014.
Letter of Credit [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	500,000,000	500,000,000
Letters of Credit Outstanding, Amount	$ 23,000,000	$ 23,000,000
Primary Commitment Holders [Member] | Revolving Credit And Letter Of Credit Facility [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Number of Participating Banks	3	3
Line of Credit Facility, Individual Bank Percentage Committment	12.00%	12.00%
Minor Commitment Holders [Member] | Revolving Credit And Letter Of Credit Facility [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Number of Participating Banks	13	13
Line of Credit Facility, Collective Bank Percentage Committment	64.00%	64.00%