Fair Value Measurements (Details) - Recurring - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Assets
Fair value, assets	$ 1,349	$ 1,693
Liabilities
Fair value, liabilities	(70)	(83)
Net assets (liabilities)	1,279	1,610
Commodity contracts | Derivative Liabilities
Liabilities
Fair value, liabilities	0	(4)
FTRs | Derivative Liabilities
Liabilities
Fair value, liabilities	(5)	0
NUG contracts | Derivative Liabilities
Liabilities
Fair value, liabilities	(65)	(79)
Corporate debt securities
Assets
Fair value, assets	474	476
FTRs | Derivative Assets
Assets
Fair value, assets	5	3
Equity securities
Assets
Fair value, assets	298	297
Foreign government debt securities
Assets
Fair value, assets	16	23
U.S. government debt securities
Assets
Fair value, assets	19	21
U.S. state debt securities
Assets
Fair value, assets	248	247
Other
Assets
Fair value, assets	289	626
Level 1
Assets
Fair value, assets	554	885
Liabilities
Fair value, liabilities	0	0
Net assets (liabilities)	554	885
Level 1 | Commodity contracts | Derivative Liabilities
Liabilities
Fair value, liabilities	0	0
Level 1 | FTRs | 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	298	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	256	588
Level 2
Assets
Fair value, assets	790	805
Liabilities
Fair value, liabilities	0	(4)
Net assets (liabilities)	790	801
Level 2 | Commodity contracts | Derivative Liabilities
Liabilities
Fair value, liabilities	0	(4)
Level 2 | FTRs | Derivative Liabilities
Liabilities
Fair value, liabilities	0	0
Level 2 | NUG contracts | Derivative Liabilities
Liabilities
Fair value, liabilities	0	0
Level 2 | Corporate debt securities
Assets
Fair value, assets	474	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	16	23
Level 2 | U.S. government debt securities
Assets
Fair value, assets	19	21
Level 2 | U.S. state debt securities
Assets
Fair value, assets	248	247
Level 2 | Other
Assets
Fair value, assets	33	38
Level 3
Assets
Fair value, assets	5	3
Liabilities
Fair value, liabilities	(70)	(79)
Net assets (liabilities)	(65)	(76)
Level 3 | Commodity contracts | Derivative Liabilities
Liabilities
Fair value, liabilities	0	0
Level 3 | FTRs | Derivative Liabilities
Liabilities
Fair value, liabilities	(5)	0
Level 3 | NUG contracts | Derivative Liabilities
Liabilities
Fair value, liabilities	(65)	(79)
Level 3 | Corporate debt securities
Assets
Fair value, assets	0	0
Level 3 | FTRs | Derivative Assets
Assets
Fair value, assets	5	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