Regulatory Accounting (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Regulatory Assets and Liabilities, Other Disclosures [Abstract]
Amount of allowed equity return on the true-up balance that has not been recognized	$ 312		$ 312
Amount of allowed equity return on the true-up balance that was recognized in the period	$ 10	$ 17	$ 17	$ 30