Regulatory Assets and Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Regulatory Assets and Liabilities Disclosure [Abstract]
Regulatory Assets Earning a Return, Other Category	$ 29	$ 33
Regulatory Assets Earning a Return	30
Regulatory Assets Earning a Rate of Return at the Cost of Capital	$ 1
Authorized Cost of Capital	7.56%
Regulatory Assets Earning a Rate of Return by Inclusion in Rate Base	$ 25
Regulatory Assets Earning a Rate of Return at the Approved Rate	$ 4
Approved Rate for Deferred Account Under Amortization, Low End of Range	1.47%
Approved Rate for Deferred Accounts under Amortization, High End of Range	1.93%