Regulatory Assets and Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Regulatory Assets and Liabilities Disclosure [Abstract]
Regulatory Assets Earning a Return, Other Category	$ 44	$ 29
Regulatory Assets Earning a Return	44
Regulatory Assets Earning a Rate of Return at the Cost of Capital	$ 19
Authorized Cost of Capital	7.56%
Regulatory Assets Earning a Rate of Return by Inclusion in Rate Base	$ 22
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	2.20%