Fair Value Measurements (Details) (USD $)	12 Months Ended
	Dec. 31, 2011 M	Dec. 31, 2010
Assets
Risk management activities: commodity contracts	$ 79,586,000	$ 112,820,000
Nuclear decommissioning trust	513,733,000	469,886,000
Liabilities
Risk management activities: commodity contracts	(136,463,000)	(124,366,000)
Cash equivalents maximum maturity period (in days)	3
Changes in fair value for assets and liabilities that are measured at fair value on a recurring basis using Level 3 inputs
Net risk management activities at beginning of period	(38,000,000)	(10,000,000)
Total net gains (losses) realized/unrealized:
Included in earnings	2,000,000	(1,000,000)
Included in OCI	(5,000,000)	(14,000,000)
Deferred as a regulatory asset or liability	(10,000,000)	(38,000,000)
Settlements	11,000,000	19,000,000
Transfers into Level 3 from Level 2	(4,000,000)	5,000,000
Transfers from Level 3 into Level 2	(7,000,000)	1,000,000
Net risk management activities at end of period	(51,000,000)	(38,000,000)
Net unrealized gains (losses) included in earnings related to instruments still held at end of period	1,000,000	(1,000,000)
Fair value measurement on a recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets
Cash equivalents		35,000,000
Nuclear decommissioning trust	69,000,000	50,000,000
Total assets	69,000,000	85,000,000
Liabilities
Risk management activities: commodity contracts		(1,000,000)
Fair value measurement on a recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. Treasury
Assets
Nuclear decommissioning trust	69,000,000	50,000,000
Fair value measurement on a recurring basis | Significant Other Observable Inputs (Level 2)
Assets
Risk management activities: commodity contracts	70,000,000	80,000,000
Nuclear decommissioning trust	446,000,000	430,000,000
Total assets	516,000,000	510,000,000
Liabilities
Risk management activities: commodity contracts	(241,000,000)	(280,000,000)
Fair value measurement on a recurring basis | Significant Other Observable Inputs (Level 2) | US commingled equity funds
Assets
Nuclear decommissioning trust	175,000,000	168,000,000
Fair value measurement on a recurring basis | Significant Other Observable Inputs (Level 2) | Cash and cash equivalent funds
Assets
Nuclear decommissioning trust	9,000,000	22,000,000
Fair value measurement on a recurring basis | Significant Other Observable Inputs (Level 2) | Corporate debt
Assets
Nuclear decommissioning trust	73,000,000	60,000,000
Fair value measurement on a recurring basis | Significant Other Observable Inputs (Level 2) | Mortgage-backed securities
Assets
Nuclear decommissioning trust	78,000,000	81,000,000
Fair value measurement on a recurring basis | Significant Other Observable Inputs (Level 2) | Municipality bonds
Assets
Nuclear decommissioning trust	90,000,000	79,000,000
Fair value measurement on a recurring basis | Significant Other Observable Inputs (Level 2) | Other
Assets
Nuclear decommissioning trust	21,000,000	20,000,000
Fair value measurement on a recurring basis | Significant Unobservable Inputs (Level 3)
Assets
Risk management activities: commodity contracts	74,000,000	61,000,000
Total assets	74,000,000	61,000,000
Liabilities
Risk management activities: commodity contracts	(125,000,000)	(99,000,000)
Fair value measurement on a recurring basis | Other
Assets
Risk management activities: commodity contracts	(64,000,000)	(28,000,000)
Nuclear decommissioning trust	(1,000,000)	(10,000,000)
Total assets	(65,000,000)	(38,000,000)
Liabilities
Risk management activities: commodity contracts	229,000,000	256,000,000
Fair value measurement on a recurring basis | Other | Cash and cash equivalent funds
Assets
Nuclear decommissioning trust	(1,000,000)
Fair value measurement on a recurring basis | Other | Other
Assets
Nuclear decommissioning trust		(10,000,000)
Fair value measurement on a recurring basis | Fair Value
Assets
Cash equivalents		35,000,000
Risk management activities: commodity contracts	80,000,000	113,000,000
Nuclear decommissioning trust	514,000,000	470,000,000
Total assets	594,000,000	618,000,000
Liabilities
Risk management activities: commodity contracts	(137,000,000)	(124,000,000)
Fair value measurement on a recurring basis | Fair Value | US commingled equity funds
Assets
Nuclear decommissioning trust	175,000,000	168,000,000
Fair value measurement on a recurring basis | Fair Value | U.S. Treasury
Assets
Nuclear decommissioning trust	69,000,000	50,000,000
Fair value measurement on a recurring basis | Fair Value | Cash and cash equivalent funds
Assets
Nuclear decommissioning trust	8,000,000	22,000,000
Fair value measurement on a recurring basis | Fair Value | Corporate debt
Assets
Nuclear decommissioning trust	73,000,000	60,000,000
Fair value measurement on a recurring basis | Fair Value | Mortgage-backed securities
Assets
Nuclear decommissioning trust	78,000,000	81,000,000
Fair value measurement on a recurring basis | Fair Value | Municipality bonds
Assets
Nuclear decommissioning trust	90,000,000	79,000,000
Fair value measurement on a recurring basis | Fair Value | Other
Assets
Nuclear decommissioning trust	$ 21,000,000	$ 10,000,000