Income Taxes - Components of Net Deferred Income Tax Liability (Details) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Deferred income tax assets: [Abstract]
Federal, state and foreign carryforwards	$ 865	$ 781
Regulatory liabilities	834	812
AROs	317	249
Employee benefits	190	187
Derivative contracts	83	62
Other	815	781
Total deferred income tax assets	3,104	2,872
Valuation allowances	(35)	(23)
Total deferred income tax assets, net	3,069	2,849
Deferred income tax liabilities: [ Abstract]
Property-related items	(13,157)	(11,989)
Regulatory assets	(1,446)	(1,374)
Investments	(852)	(699)
Other	(299)	(301)
Total deferred income tax liabilities	(15,754)	(14,363)
Net deferred income tax liability	$ (12,685)	$ (11,514)