Fair Value Measurements - Recurring Assets and Liabilities (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Liabilities
Investment excludes receivables, payables and accrued income		$ 1
Recurring
Assets
Fair value, assets	$ 1,010	2,123
Liabilities
Fair value, liabilities	(1)	0
Net assets (liabilities)	1,009	2,123
Recurring | FTRs | Derivative Liabilities
Liabilities
Fair value, liabilities	(1)	0
Recurring | FTRs | Derivative Assets
Assets
Fair value, assets	10	3
Recurring | Equity securities
Assets
Fair value, assets	2	2
Recurring | U.S. state debt securities
Assets
Fair value, assets	271	276
Recurring | Cash, cash equivalents and restricted cash
Assets
Fair value, assets	684	1,801
Recurring | Other
Assets
Fair value, assets	43	41
Recurring | Level 1
Assets
Fair value, assets	686	1,803
Liabilities
Fair value, liabilities	0	0
Net assets (liabilities)	686	1,803
Recurring | Level 1 | FTRs | Derivative Liabilities
Liabilities
Fair value, liabilities	0	0
Recurring | Level 1 | FTRs | Derivative Assets
Assets
Fair value, assets	0	0
Recurring | Level 1 | Equity securities
Assets
Fair value, assets	2	2
Recurring | Level 1 | U.S. state debt securities
Assets
Fair value, assets	0	0
Recurring | Level 1 | Cash, cash equivalents and restricted cash
Assets
Fair value, assets	684	1,801
Recurring | Level 1 | Other
Assets
Fair value, assets	0	0
Recurring | Level 2
Assets
Fair value, assets	314	317
Liabilities
Fair value, liabilities	0	0
Net assets (liabilities)	314	317
Recurring | Level 2 | FTRs | Derivative Liabilities
Liabilities
Fair value, liabilities	0	0
Recurring | Level 2 | FTRs | Derivative Assets
Assets
Fair value, assets	0	0
Recurring | Level 2 | Equity securities
Assets
Fair value, assets	0	0
Recurring | Level 2 | U.S. state debt securities
Assets
Fair value, assets	271	276
Recurring | Level 2 | Cash, cash equivalents and restricted cash
Assets
Fair value, assets	0	0
Recurring | Level 2 | Other
Assets
Fair value, assets	43	41
Recurring | Level 3
Assets
Fair value, assets	10	3
Liabilities
Fair value, liabilities	(1)	0
Net assets (liabilities)	9	3
Recurring | Level 3 | FTRs | Derivative Liabilities
Liabilities
Fair value, liabilities	(1)	0
Recurring | Level 3 | FTRs | Derivative Assets
Assets
Fair value, assets	10	3
Recurring | Level 3 | Equity securities
Assets
Fair value, assets	0	0
Recurring | Level 3 | U.S. state debt securities
Assets
Fair value, assets	0	0
Recurring | Level 3 | Cash, cash equivalents and restricted cash
Assets
Fair value, assets	0	0
Recurring | Level 3 | Other
Assets
Fair value, assets	$ 0	$ 0