Regulatory Assets and Liabilities (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Regulatory Assets and Liabilities Disclosure [Abstract]
Regulatory Assets Earning a Return, Other Category	$ 15	$ 21
Regulatory Assets Earning a Return	31
Regulatory Assets Earning a Rate of Return at the Cost of Capital	18
Authorized Cost of Capital	6.065%
Regulatory Assets Earning a Rate of Return at the Approved Rate	3
Approved Rate for Deferred Account Under Amortization, Low End of Range	1.47%
Approved Rate for Deferred Accounts under Amortization, High End of Range	2.24%
Regulatory Assets Earning a Rate of Return by Inclusion in Rate Base	$ 10