Fair Value Measurements - Recurring Assets and Liabilities (Details) - Recurring - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Assets
Fair value, assets	$ 1,596	$ 1,438
Liabilities
Fair value, liabilities	(39)	(45)
Net assets (liabilities)	1,557	1,393
FTRs | Derivative Liabilities
Liabilities
Fair value, liabilities	(4)	(1)
NUG contracts | Derivative Liabilities
Liabilities
Fair value, liabilities	(35)	(44)
Corporate debt securities
Assets
Fair value, assets	439	405
FTRs | Derivative Assets
Assets
Fair value, assets	6	10
Equity securities
Assets
Fair value, assets	395	339
Foreign government debt securities
Assets
Fair value, assets	16	13
U.S. government debt securities
Assets
Fair value, assets	19	20
U.S. state debt securities
Assets
Fair value, assets	263	250
Other
Assets
Fair value, assets	458	401
Level 1
Assets
Fair value, assets	817	706
Liabilities
Fair value, liabilities	0	0
Net assets (liabilities)	817	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	395	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	422	367
Level 2
Assets
Fair value, assets	773	722
Liabilities
Fair value, liabilities	0	0
Net assets (liabilities)	773	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	439	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	16	13
Level 2 | U.S. government debt securities
Assets
Fair value, assets	19	20
Level 2 | U.S. state debt securities
Assets
Fair value, assets	263	250
Level 2 | Other
Assets
Fair value, assets	36	34
Level 3
Assets
Fair value, assets	6	10
Liabilities
Fair value, liabilities	(39)	(45)
Net assets (liabilities)	(33)	(35)
Level 3 | FTRs | Derivative Liabilities
Liabilities
Fair value, liabilities	(4)	(1)
Level 3 | NUG contracts | Derivative Liabilities
Liabilities
Fair value, liabilities	(35)	(44)
Level 3 | Corporate debt securities
Assets
Fair value, assets	0	0
Level 3 | FTRs | Derivative Assets
Assets
Fair value, assets	6	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