Regulatory Assets and Liabilities (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Regulatory Assets and Liabilities Disclosure [Abstract]
Regulatory Assets Earning a Return, Other Category	$ 51	$ 44
Regulatory Assets Earning a Return	51
Regulatory Assets Earning a Rate of Return at the Cost of Capital	$ 10
Public Utilities, Approved Return on Equity, Percentage	7.51%
Authorized Cost of Capital	2.87%
Regulatory Assets Earning a Rate of Return by Inclusion in Rate Base	$ 14
Regulatory Assets Earning a Rate of Return at the Approved Rate	$ 25
Approved Rate for Deferred Account Under Amortization, Low End of Range	1.47%
Approved Rate for Deferred Accounts under Amortization, High End of Range	2.38%
Types of Net Regulatory Assets Earning Returns	$ 2
Increase in net deferred tax regulatory liabilitily	$ 357