Fair Value Measurements (Details) - Recurring - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Assets
Fair value, assets	$ 1,571	$ 1,693
Liabilities
Fair value, liabilities	(53)	(79)
Net assets (liabilities)	1,518	1,614
FTRs | Derivative Liabilities
Liabilities
Fair value, liabilities	(1)	0
NUG contracts | Derivative Liabilities
Liabilities
Fair value, liabilities	(52)	(79)
Corporate debt securities
Assets
Fair value, assets	405	476
FTRs | Derivative Assets
Assets
Fair value, assets	10	3
Equity securities
Assets
Fair value, assets	329	297
Foreign government debt securities
Assets
Fair value, assets	12	23
U.S. government debt securities
Assets
Fair value, assets	22	21
U.S. state debt securities
Assets
Fair value, assets	251	247
Other
Assets
Fair value, assets	542	626
Level 1
Assets
Fair value, assets	765	885
Liabilities
Fair value, liabilities	0	0
Net assets (liabilities)	765	885
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	329	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	436	588
Level 2
Assets
Fair value, assets	796	805
Liabilities
Fair value, liabilities	0	0
Net assets (liabilities)	796	805
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	405	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	12	23
Level 2 | U.S. government debt securities
Assets
Fair value, assets	22	21
Level 2 | U.S. state debt securities
Assets
Fair value, assets	251	247
Level 2 | Other
Assets
Fair value, assets	106	38
Level 3
Assets
Fair value, assets	10	3
Liabilities
Fair value, liabilities	(53)	(79)
Net assets (liabilities)	(43)	(76)
Level 3 | FTRs | Derivative Liabilities
Liabilities
Fair value, liabilities	(1)	0
Level 3 | NUG contracts | Derivative Liabilities
Liabilities
Fair value, liabilities	(52)	(79)
Level 3 | Corporate debt securities
Assets
Fair value, assets	0	0
Level 3 | FTRs | Derivative Assets
Assets
Fair value, assets	10	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