Income Taxes - Components of Net Deferred Income Tax Liability (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Deferred income tax assets: [Abstract]
Federal, state and foreign carryforwards	$ 987	$ 865
Regulatory liabilities	909	834
AROs	326	317
Employee benefits	209	190
Derivative contracts	29	83
Other	707	815
Total deferred income tax assets	3,167	3,104
Valuation allowances	(64)	(35)
Total deferred income tax assets, net	3,103	3,069
Deferred income tax liabilities: [ Abstract]
Property-related items	(14,237)	(13,157)
Regulatory assets	(1,449)	(1,446)
Investments	(962)	(852)
Other	(334)	(299)
Total deferred income tax liabilities	(16,982)	(15,754)
Net deferred income tax liability	$ (13,879)	$ (12,685)