Fair Value Measurements - Recurring Assets and Liabilities (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Liabilities
Investment excludes receivables, payables and accrued income	$ 1	$ 1
Recurring
Assets
Fair value, assets	1,596	2,056
Liabilities
Fair value, liabilities	(1)	0
Net assets (liabilities)	1,595	2,056
Recurring | FTRs | Derivative Liabilities
Liabilities
Fair value, liabilities	(1)	0
Recurring | FTRs | Derivative Assets
Assets
Fair value, assets	0	3
Recurring | Equity securities
Assets
Fair value, assets	2	2
Recurring | U.S. state debt securities
Assets
Fair value, assets	276	276
Recurring | Cash and cash equivalents
Assets
Fair value, assets	1,281	1,734
Recurring | Other
Assets
Fair value, assets	37	41
Recurring | Level 1
Assets
Fair value, assets	1,283	1,736
Liabilities
Fair value, liabilities	0	0
Net assets (liabilities)	1,283	1,736
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 and cash equivalents
Assets
Fair value, assets	1,281	1,734
Recurring | Level 1 | Other
Assets
Fair value, assets	0	0
Recurring | Level 2
Assets
Fair value, assets	313	317
Liabilities
Fair value, liabilities	0	0
Net assets (liabilities)	313	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	276	276
Recurring | Level 2 | Cash and cash equivalents
Assets
Fair value, assets	0	0
Recurring | Level 2 | Other
Assets
Fair value, assets	37	41
Recurring | Level 3
Assets
Fair value, assets	0	3
Liabilities
Fair value, liabilities	(1)	0
Net assets (liabilities)	(1)	3
Recurring | Level 3 | FTRs | Derivative Liabilities
Liabilities
Fair value, liabilities	(1)	0
Recurring | Level 3 | FTRs | Derivative Assets
Assets
Fair value, assets	0	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 and cash equivalents
Assets
Fair value, assets	0	0
Recurring | Level 3 | Other
Assets
Fair value, assets	$ 0	$ 0