Fair Value Measurements - Recurring Assets and Liabilities (Details) - USD ($) $ in Millions	Jun. 30, 2020	Dec. 31, 2019
Liabilities
Investment excludes receivables, payables and accrued income	$ (19)	$ (16)
Recurring
Assets
Fair value, assets	1,332	1,828
Liabilities
Fair value, liabilities	(2)	(17)
Net assets (liabilities)	1,330	1,811
Recurring | FTRs | Derivative Liabilities
Liabilities
Fair value, liabilities	(2)	(1)
Recurring | NUG contracts | Derivative Liabilities
Liabilities
Fair value, liabilities	0	(16)
Recurring | Corporate debt securities
Assets
Fair value, assets	131	135
Recurring | FTRs | Derivative Assets
Assets
Fair value, assets	7	4
Recurring | Equity securities
Assets
Fair value, assets	2	2
Recurring | U.S. state debt securities
Assets
Fair value, assets	274	271
Recurring | Other
Assets
Fair value, assets	918	1,416
Recurring | Level 1
Assets
Fair value, assets	118	629
Liabilities
Fair value, liabilities	0	0
Net assets (liabilities)	118	629
Recurring | Level 1 | FTRs | Derivative Liabilities
Liabilities
Fair value, liabilities	0	0
Recurring | Level 1 | NUG contracts | Derivative Liabilities
Liabilities
Fair value, liabilities	0	0
Recurring | Level 1 | Corporate debt securities
Assets
Fair value, assets	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 | Other
Assets
Fair value, assets	116	627
Recurring | Level 2
Assets
Fair value, assets	1,207	1,195
Liabilities
Fair value, liabilities	0	0
Net assets (liabilities)	1,207	1,195
Recurring | Level 2 | FTRs | Derivative Liabilities
Liabilities
Fair value, liabilities	0	0
Recurring | Level 2 | NUG contracts | Derivative Liabilities
Liabilities
Fair value, liabilities	0	0
Recurring | Level 2 | Corporate debt securities
Assets
Fair value, assets	131	135
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	274	271
Recurring | Level 2 | Other
Assets
Fair value, assets	802	789
Recurring | Level 3
Assets
Fair value, assets	7	4
Liabilities
Fair value, liabilities	(2)	(17)
Net assets (liabilities)	5	(13)
Recurring | Level 3 | FTRs | Derivative Liabilities
Liabilities
Fair value, liabilities	(2)	(1)
Recurring | Level 3 | NUG contracts | Derivative Liabilities
Liabilities
Fair value, liabilities	0	(16)
Recurring | Level 3 | Corporate debt securities
Assets
Fair value, assets	0	0
Recurring | Level 3 | FTRs | Derivative Assets
Assets
Fair value, assets	7	4
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 | Other
Assets
Fair value, assets	$ 0	$ 0