Income Taxes Income taxes (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010
Operating Loss Carryforwards [Line Items]
Deferred Taxes Reclass, To Depreciation and Amortization From Regulatory Liabilities		$ 220
Deferred Taxes Reclass, To Price Risk Management From Regulatory Liabilities		74
Deferred Taxes Reclass, To Employee Benefits From Regulatory Assets		73
Deferred Taxes Reclass To Regulatory Assets From Other		8
Loss Carryforward, Federal	0
Loss Carryforward, State	1
Tax Credit Carryforward, Federal	42
Tax Credit Carryforward, State	14
Deferred Tax Assets, Valuation Allowance	0	2
Valuation Allowance, Deferred Tax Asset, Change in Amount	(2)	(1)
Unrecognized Tax Benefits	0	2
Unrecognized Tax Benefits, Interest on Income Taxes Expense		1
Unrecognized Tax Benefits, Income Tax Penalties Expense		0
Unrecognized Tax Benefits, Decreases Resulting from Current Period Tax Positions	$ 2