Fair Value Measurements - Recurring Assets and Liabilities (Details) - Recurring - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Assets
Fair value, assets	$ 1,902	$ 1,438
Liabilities
Fair value, liabilities	(27)	(45)
Net assets (liabilities)	1,875	1,393
FTRs | Derivative Liabilities
Liabilities
Fair value, liabilities	(3)	(1)
NUG contracts | Derivative Liabilities
Liabilities
Fair value, liabilities	(24)	(44)
Corporate debt securities
Assets
Fair value, assets	446	405
FTRs | Derivative Assets
Assets
Fair value, assets	6	10
Equity securities
Assets
Fair value, assets	362	339
Foreign government debt securities
Assets
Fair value, assets	17	13
U.S. government debt securities
Assets
Fair value, assets	18	20
U.S. state debt securities
Assets
Fair value, assets	266	250
Other
Assets
Fair value, assets	787	401
Level 1
Assets
Fair value, assets	1,078	706
Liabilities
Fair value, liabilities	0	0
Net assets (liabilities)	1,078	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	362	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	716	367
Level 2
Assets
Fair value, assets	818	722
Liabilities
Fair value, liabilities	0	0
Net assets (liabilities)	818	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	446	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	17	13
Level 2 | U.S. government debt securities
Assets
Fair value, assets	18	20
Level 2 | U.S. state debt securities
Assets
Fair value, assets	266	250
Level 2 | Other
Assets
Fair value, assets	71	34
Level 3
Assets
Fair value, assets	6	10
Liabilities
Fair value, liabilities	(27)	(45)
Net assets (liabilities)	(21)	(35)
Level 3 | FTRs | Derivative Liabilities
Liabilities
Fair value, liabilities	(3)	(1)
Level 3 | NUG contracts | Derivative Liabilities
Liabilities
Fair value, liabilities	(24)	(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