Regulatory Matters Regulatory Liabilities (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011
Regulatory Liabilities [Line Items]
Current regulatory liabilities	$ 62		$ 67
Noncurrent regulatory liabilities	851		826
Total regulatory liabilities	913		893
Cost of removal [Member]
Regulatory Liabilities [Line Items]
Noncurrent regulatory liabilities	810		782
Total regulatory liabilities	810	[1]	782	[1]
Weighted Average Remaining Life	33 years	[1]
Deferred income taxes [Member]
Regulatory Liabilities [Line Items]
Total regulatory liabilities	21		22
Weighted Average Remaining Life	Various
Other [Member]
Regulatory Liabilities [Line Items]
Total regulatory liabilities	$ 82		$ 89
Weighted Average Remaining Life	Various

[1] Amounts represent estimated costs, as accrued through depreciation rates and exclusive of ARO liabilities, of removing property, plant and equipment in accordance with accepted regulatory practices. Amounts are deducted from rate base or otherwise accrue a carrying cost.