Fair Value Measurements (Summary of Fair Value of Derivative Assets and Liabilities and Effect of Counterparty Netting) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset, Fair Value, Gross Liability	$ (546)	$ (675)
Derivative Asset, Fair Value, Amount Offset Against Collateral		(8)
Investments, Fair Value Disclosure	64	50
Total assets	660	817
Derivative Liability, Fair Value, Gross Asset	(546)	(675)
Derivative Liability, Fair Value, Amount Offset Against Collateral	(41)	(42)
Debt Excluding Capital Leases Difference In Carrying Value And Fair Value	(262)	(590)
Total liabilities	6,852	8,324
Derivative Liabilities And Debt Instrument, Fair Value Disclosure	6,003	7,017
Reported Value Measurement [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	64	50
Derivative Assets And Investments, Fair Value Disclosure	114	134
Floating Rate Debt [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Instrument, Fair Value Disclosure	50	1,050
Debt Excluding Capital Leases Carrying Value	50	1,050
Fixed Rate Debt Excluding Capital Leases [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Instrument, Fair Value Disclosure	6,168	6,508
Debt Excluding Capital Leases Carrying Value	5,906	5,918
Debt Excluding Capital Leases Difference In Carrying Value And Fair Value	(262)	(590)
Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	64	50
Total assets	291	430
Total liabilities	303	1,443
Level 1 [Member] | Floating Rate Debt [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Instrument, Fair Value Disclosure	50	1,050
Level 1 [Member] | Fixed Rate Debt Excluding Capital Leases [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Instrument, Fair Value Disclosure	0	0
Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	0	0
Total assets	367	385
Total liabilities	6,548	6,880
Level 2 [Member] | Floating Rate Debt [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Instrument, Fair Value Disclosure	0	0
Level 2 [Member] | Fixed Rate Debt Excluding Capital Leases [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Instrument, Fair Value Disclosure	6,168	6,508
Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	0	0
Total assets	2	2
Total liabilities	1	1
Level 3 [Member] | Floating Rate Debt [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Instrument, Fair Value Disclosure	0	0
Level 3 [Member] | Fixed Rate Debt Excluding Capital Leases [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Instrument, Fair Value Disclosure	0	0
Exchange Cleared [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Price Risk Derivative Instruments Not Designated as Hedging Instruments Asset, at Fair Value	559	689
Derivative Asset, Fair Value, Gross Liability	538	672
Derivative Asset, Fair Value, Amount Offset Against Collateral	0	8
Derivative Asset	21	9
Price Risk Derivative Instruments Not Designated as Hedging Instruments Liability, at Fair Value	579	721
Derivative Liability, Fair Value, Gross Asset	538	672
Derivative Liability, Fair Value, Amount Offset Against Collateral	41	42
Derivative Liability		7
Derivative, Collateral, Right to Reclaim Cash		(7)
Exchange Cleared [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Price Risk Derivative Instruments Not Designated as Hedging Instruments Asset, at Fair Value	227	380
Price Risk Derivative Instruments Not Designated as Hedging Instruments Liability, at Fair Value	253	393
Exchange Cleared [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Price Risk Derivative Instruments Not Designated as Hedging Instruments Asset, at Fair Value	332	309
Price Risk Derivative Instruments Not Designated as Hedging Instruments Liability, at Fair Value	326	328
Exchange Cleared [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Price Risk Derivative Instruments Not Designated as Hedging Instruments Asset, at Fair Value	0	0
Price Risk Derivative Instruments Not Designated as Hedging Instruments Liability, at Fair Value	0	0
Over the Counter [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Price Risk Derivative Instruments Not Designated as Hedging Instruments Asset, at Fair Value	10	15
Derivative Asset, Fair Value, Gross Liability	8	7
Derivative Asset	2	8
Price Risk Derivative Instruments Not Designated as Hedging Instruments Liability, at Fair Value	11	13
Derivative Liability, Fair Value, Gross Asset	8	7
Derivative Liability	3	6
Over the Counter [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Price Risk Derivative Instruments Not Designated as Hedging Instruments Asset, at Fair Value	0	0
Price Risk Derivative Instruments Not Designated as Hedging Instruments Liability, at Fair Value	0	0
Over the Counter [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Price Risk Derivative Instruments Not Designated as Hedging Instruments Asset, at Fair Value	10	15
Price Risk Derivative Instruments Not Designated as Hedging Instruments Liability, at Fair Value	11	13
Over the Counter [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Price Risk Derivative Instruments Not Designated as Hedging Instruments Asset, at Fair Value	0	0
Price Risk Derivative Instruments Not Designated as Hedging Instruments Liability, at Fair Value	0	0
Forward Contracts [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Price Risk Derivative Instruments Not Designated as Hedging Instruments Asset, at Fair Value	27	63
Derivative Asset, Fair Value, Gross Liability	0	(4)
Derivative Asset	27	67
Price Risk Derivative Instruments Not Designated as Hedging Instruments Liability, at Fair Value	44	32
Derivative Liability, Fair Value, Gross Asset	0	(4)
Derivative Liability	44	36
Forward Contracts [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Price Risk Derivative Instruments Not Designated as Hedging Instruments Asset, at Fair Value	0	0
Price Risk Derivative Instruments Not Designated as Hedging Instruments Liability, at Fair Value	0	0
Forward Contracts [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Price Risk Derivative Instruments Not Designated as Hedging Instruments Asset, at Fair Value	25	61
Price Risk Derivative Instruments Not Designated as Hedging Instruments Liability, at Fair Value	43	31
Forward Contracts [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Price Risk Derivative Instruments Not Designated as Hedging Instruments Asset, at Fair Value	2	2
Price Risk Derivative Instruments Not Designated as Hedging Instruments Liability, at Fair Value	$ 1	$ 1