Fair Value of Financial Instruments Fair Value of Financial Instruments (Details) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013	Dec. 31, 2013
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Net gain or loss recognized in the statement of income offset by regulatory accounting	$ 52,000,000	$ (56,000,000)	$ 64,000,000	$ (59,000,000)
Defined Benefit Plan, Transfers Between Measurement Levels	0	0	0	0
Cash Surrender Value, Fair Value Disclosure	26,000,000		26,000,000		26,000,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3	0	0	0	0
Long-term Debt	2,141,000,000		2,141,000,000		1,916,000,000
Long-term Debt, Fair Value	2,408,000,000		2,408,000,000		2,074,000,000
Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term Debt, Fair Value	2,183,000,000		2,183,000,000
Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term Debt, Fair Value	$ 225,000,000		$ 225,000,000