Fair Value Measurements (Details) - Recurring - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Assets
Fair value, assets	$ 1,828	$ 1,438
Liabilities
Fair value, liabilities	(17)	(45)
Net assets (liabilities)	1,811	1,393
FTRs | Derivative Liabilities
Liabilities
Fair value, liabilities	(1)	(1)
NUG contracts | Derivative Liabilities
Liabilities
Fair value, liabilities	(16)	(44)
Corporate debt securities
Assets
Fair value, assets	135	405
FTRs | Derivative Assets
Assets
Fair value, assets	4	10
Equity securities
Assets
Fair value, assets	2	339
Foreign government debt securities
Assets
Fair value, assets	0	13
U.S. government debt securities
Assets
Fair value, assets	0	20
U.S. state debt securities
Assets
Fair value, assets	271	250
Other
Assets
Fair value, assets	1,416	401
Level 1
Assets
Fair value, assets	629	706
Liabilities
Fair value, liabilities	0	0
Net assets (liabilities)	629	706
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	2	339
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	627	367
Level 2
Assets
Fair value, assets	1,195	722
Liabilities
Fair value, liabilities	0	0
Net assets (liabilities)	1,195	722
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	135	405
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	0	13
Level 2 | U.S. government debt securities
Assets
Fair value, assets	0	20
Level 2 | U.S. state debt securities
Assets
Fair value, assets	271	250
Level 2 | Other
Assets
Fair value, assets	789	34
Level 3
Assets
Fair value, assets	4	10
Liabilities
Fair value, liabilities	(17)	(45)
Net assets (liabilities)	(13)	(35)
Level 3 | FTRs | Derivative Liabilities
Liabilities
Fair value, liabilities	(1)	(1)
Level 3 | NUG contracts | Derivative Liabilities
Liabilities
Fair value, liabilities	(16)	(44)
Level 3 | Corporate debt securities
Assets
Fair value, assets	0	0
Level 3 | FTRs | Derivative Assets
Assets
Fair value, assets	4	10
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