Risk Management Activities and Derivatives Transactions (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011 years MMBTU gal	Dec. 31, 2010	Dec. 31, 2009	Feb. 15, 2012
Economic derivatives
Cash collateral posted	$ 147	$ 164
Notional MMBtu of natural gas	380,000,000
Notional gallons of oil	10,300,000
Interest rate cash flow hedges
Open interest rate hedges mandatory termination dates	2
After-tax (gains) losses in accumulated other comprehensive income	141	63	35
Next 12 months amount of (gains) losses to reclass to interest expense	12
Notional amount of interest rate cash flow hedges	500	1,050
Contingent features
Aggregate fair value of instruments in liability position	489
Additional collateral to post	342
Derivatives Fair Value [Line Items]
Derivative asset fair value	5	19
Derivative liability fair value	802	552
Derivatives Designated as Hedging Instruments
Amount of Gain or (Loss) Recognized in OCI, Net of Tax on Derivatives	(87)	(34)	16
Amount of Gain or (Loss), Net of Tax Reclassified from Accumulated OCI into Income	(8)	(6)	(6)
Amount of Pre-tax Gain or (Loss) Recognized in Income on Derivatives	(3)	3	(3)
Derivatives Not Designated As Hedging Instruments
Amount of Gain or (Loss) Recognized in Income on Derivatives	(58)	1	22
Fair Value Of Derivative Instruments Parenthetical Information
CVO's in connection with the acquisition of Florida Progress during 2000	98.6
CVO's repurchased in connection with the negotiated settlement agreement and subsequent tender offer	80.1			83.4
Original fair value loss transition adjustment, pre-tax, recorded in 2003	38
Original fair value loss transition adjustment, after-tax, recorded in 2003	23
Derivatives designated as hedging instruments
Derivatives Fair Value [Line Items]
Derivative asset fair value		4
Derivative liability fair value	96	39
Derivatives not designated as hedging instruments
Derivatives Fair Value [Line Items]
Derivative asset fair value	5	15
Derivative liability fair value	706	513
Fair value of derivatives not designated as hedging instruments | Derivatives not designated as hedging instruments
Derivatives Fair Value [Line Items]
Derivative asset fair value	5	15
Derivative liability fair value	703	509
Interest rate derivatives
Derivatives Designated as Hedging Instruments
Amount of Gain or (Loss) Recognized in OCI, Net of Tax on Derivatives	(85)	(34)	15
Amount of Gain or (Loss), Net of Tax Reclassified from Accumulated OCI into Income	(8)	(6)	(6)
Amount of Pre-tax Gain or (Loss) Recognized in Income on Derivatives	(3)	3	(3)
Interest rate derivatives | Prepayments and other current assets | Derivatives designated as hedging instruments
Derivatives Fair Value [Line Items]
Derivative asset fair value		1
Interest rate derivatives | Other assets and deferred debits | Derivatives designated as hedging instruments
Derivatives Fair Value [Line Items]
Derivative asset fair value		3
Interest rate derivatives | Derivative liabilities, current | Derivatives designated as hedging instruments
Derivatives Fair Value [Line Items]
Derivative liability fair value	76	32
Interest rate derivatives | Derivative liabilities, long-term | Derivatives designated as hedging instruments
Derivatives Fair Value [Line Items]
Derivative liability fair value	17	7
Commodity derivatives
Derivatives Designated as Hedging Instruments
Amount of Gain or (Loss) Recognized in OCI, Net of Tax on Derivatives	(2)		1
Derivatives Not Designated As Hedging Instruments
Realized Gain or (Loss)	(297)	(324)	(659)
Unrealized Gain or (Loss)	(502)	(398)	(387)
Amount of Gain or (Loss) Recognized in Income on Derivatives			1
Commodity derivatives | Prepayments and other current assets | Derivatives not designated as hedging instruments
Derivatives Fair Value [Line Items]
Derivative asset fair value	5	11
Commodity derivatives | Other assets and deferred debits | Derivatives not designated as hedging instruments
Derivatives Fair Value [Line Items]
Derivative asset fair value		4
Commodity derivatives | Derivative liabilities, current | Derivatives designated as hedging instruments
Derivatives Fair Value [Line Items]
Derivative liability fair value	2
Commodity derivatives | Derivative liabilities, current | Derivatives not designated as hedging instruments
Derivatives Fair Value [Line Items]
Derivative liability fair value	357	226
Commodity derivatives | Derivative liabilities, long-term | Derivatives designated as hedging instruments
Derivatives Fair Value [Line Items]
Derivative liability fair value	1
Commodity derivatives | Derivative liabilities, long-term | Derivatives not designated as hedging instruments
Derivatives Fair Value [Line Items]
Derivative liability fair value	332	268
CVOs
Derivatives Not Designated As Hedging Instruments
Amount of Gain or (Loss) Recognized in Income on Derivatives	(59)		19
CVOs | Prepayments and other current assets | Derivatives not designated as hedging instruments
Derivatives Fair Value [Line Items]
Derivative liability fair value	14
CVOs | Other assets and deferred debits | Derivatives not designated as hedging instruments
Derivatives Fair Value [Line Items]
Derivative liability fair value		15
Fair value loss transition adjustment
Derivatives Not Designated As Hedging Instruments
Amount of Gain or (Loss) Recognized in Income on Derivatives	1	1	2
Fair value loss transition adjustment | Derivative liabilities, current | Derivatives not designated as hedging instruments
Derivatives Fair Value [Line Items]
Derivative liability fair value	1	1
Fair value loss transition adjustment | Derivative liabilities, long-term | Derivatives not designated as hedging instruments
Derivatives Fair Value [Line Items]
Derivative liability fair value	$ 2	$ 3