FAIR VALUE MEASUREMENTS - Assets and Liabilities Measured on Recurring Basis (Details) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023
Liabilities
Restricted cash	$ 27	$ 42
Recurring
Assets
Total assets	1,237	500
Liabilities
Total liabilities	(1)	(1)
Net assets (liabilities)	1,236	499
Recurring | FTRs | Derivative Liabilities
Liabilities
Total liabilities	(1)	(1)
Recurring | FTRs | Derivative Assets
Assets
Total assets	0	4
Recurring | Equity securities
Assets
Total assets	2	2
Recurring | U.S. state and municipal debt securities
Assets
Total assets	274	275
Recurring | Cash, cash equivalents and restricted cash
Assets
Total assets	915	179
Recurring | Other
Assets
Total assets	46	40
Recurring | Level 1
Assets
Total assets	917	181
Liabilities
Total liabilities	0	0
Net assets (liabilities)	917	181
Recurring | Level 1 | FTRs | Derivative Liabilities
Liabilities
Total liabilities	0	0
Recurring | Level 1 | FTRs | Derivative Assets
Assets
Total assets	0	0
Recurring | Level 1 | Equity securities
Assets
Total assets	2	2
Recurring | Level 1 | U.S. state and municipal debt securities
Assets
Total assets	0	0
Recurring | Level 1 | Cash, cash equivalents and restricted cash
Assets
Total assets	915	179
Recurring | Level 1 | Other
Assets
Total assets	0	0
Recurring | Level 2
Assets
Total assets	320	315
Liabilities
Total liabilities	0	0
Net assets (liabilities)	320	315
Recurring | Level 2 | FTRs | Derivative Liabilities
Liabilities
Total liabilities	0	0
Recurring | Level 2 | FTRs | Derivative Assets
Assets
Total assets	0	0
Recurring | Level 2 | Equity securities
Assets
Total assets	0	0
Recurring | Level 2 | U.S. state and municipal debt securities
Assets
Total assets	274	275
Recurring | Level 2 | Cash, cash equivalents and restricted cash
Assets
Total assets	0	0
Recurring | Level 2 | Other
Assets
Total assets	46	40
Recurring | Level 3
Assets
Total assets	0	4
Liabilities
Total liabilities	(1)	(1)
Net assets (liabilities)	(1)	3
Recurring | Level 3 | FTRs | Derivative Liabilities
Liabilities
Total liabilities	(1)	(1)
Recurring | Level 3 | FTRs | Derivative Assets
Assets
Total assets	0	4
Recurring | Level 3 | Equity securities
Assets
Total assets	0	0
Recurring | Level 3 | U.S. state and municipal debt securities
Assets
Total assets	0	0
Recurring | Level 3 | Cash, cash equivalents and restricted cash
Assets
Total assets	0	0
Recurring | Level 3 | Other
Assets
Total assets	$ 0	$ 0