Fair Value Measurements (Details) - Recurring - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Assets
Fair value, assets	$ 1,332	$ 1,693
Liabilities
Fair value, liabilities	(74)	(83)
Net assets (liabilities)	1,258	1,610
Commodity contracts | Derivative Liabilities
Liabilities
Fair value, liabilities	0	(4)
NUG contracts | Derivative Liabilities
Liabilities
Fair value, liabilities	(74)	(79)
Corporate debt securities
Assets
Fair value, assets	468	476
FTRs | Derivative Assets
Assets
Fair value, assets	1	3
Equity securities
Assets
Fair value, assets	288	297
Foreign government debt securities
Assets
Fair value, assets	24	23
U.S. government debt securities
Assets
Fair value, assets	28	21
U.S. state debt securities
Assets
Fair value, assets	245	247
Other
Assets
Fair value, assets	278	626
Level 1
Assets
Fair value, assets	536	885
Liabilities
Fair value, liabilities	0	0
Net assets (liabilities)	536	885
Level 1 | Commodity contracts | Derivative Liabilities
Liabilities
Fair value, liabilities	0	0
Level 1 | NUG contracts | Derivative Liabilities
Liabilities
Fair value, liabilities	0	0
Level 1 | Corporate debt securities
Assets
Fair value, assets	0	0
Level 1 | FTRs | Derivative Assets
Assets
Fair value, assets	0	0
Level 1 | Equity securities
Assets
Fair value, assets	288	297
Level 1 | Foreign government debt securities
Assets
Fair value, assets	0	0
Level 1 | U.S. government debt securities
Assets
Fair value, assets	0	0
Level 1 | U.S. state debt securities
Assets
Fair value, assets	0	0
Level 1 | Other
Assets
Fair value, assets	248	588
Level 2
Assets
Fair value, assets	795	805
Liabilities
Fair value, liabilities	0	(4)
Net assets (liabilities)	795	801
Level 2 | Commodity contracts | Derivative Liabilities
Liabilities
Fair value, liabilities	0	(4)
Level 2 | NUG contracts | Derivative Liabilities
Liabilities
Fair value, liabilities	0	0
Level 2 | Corporate debt securities
Assets
Fair value, assets	468	476
Level 2 | FTRs | Derivative Assets
Assets
Fair value, assets	0	0
Level 2 | Equity securities
Assets
Fair value, assets	0	0
Level 2 | Foreign government debt securities
Assets
Fair value, assets	24	23
Level 2 | U.S. government debt securities
Assets
Fair value, assets	28	21
Level 2 | U.S. state debt securities
Assets
Fair value, assets	245	247
Level 2 | Other
Assets
Fair value, assets	30	38
Level 3
Assets
Fair value, assets	1	3
Liabilities
Fair value, liabilities	(74)	(79)
Net assets (liabilities)	(73)	(76)
Level 3 | Commodity contracts | Derivative Liabilities
Liabilities
Fair value, liabilities	0	0
Level 3 | NUG contracts | Derivative Liabilities
Liabilities
Fair value, liabilities	(74)	(79)
Level 3 | Corporate debt securities
Assets
Fair value, assets	0	0
Level 3 | FTRs | Derivative Assets
Assets
Fair value, assets	1	3
Level 3 | Equity securities
Assets
Fair value, assets	0	0
Level 3 | Foreign government debt securities
Assets
Fair value, assets	0	0
Level 3 | U.S. government debt securities
Assets
Fair value, assets	0	0
Level 3 | U.S. state debt securities
Assets
Fair value, assets	0	0
Level 3 | Other
Assets
Fair value, assets	$ 0	$ 0