Consolidated Statements of Changes in Common Equity (Unaudited) - USD ($) shares in Millions, $ in Millions	Total	Cumulative Effect, Period of Adoption, Adjustment	Common Stock	Retained Earnings	Retained Earnings Cumulative Effect, Period of Adoption, Adjustment	AOCI	AOCI Cumulative Effect, Period of Adoption, Adjustment	Noncontrolling Interest
Beginning balance at Dec. 31, 2018	$ 4,315	$ 0	$ 2,860	$ 1,279	$ 1	$ (3)	$ (1)	$ 179
Beginning balance (in shares) at Dec. 31, 2018			40
Accounting standards update [extensible list]				us-gaap:AccountingStandardsUpdate201802Member		us-gaap:AccountingStandardsUpdate201802Member
Total comprehensive income (loss) available (attributable) to common shareholder	(1,030)			$ (1,045)		$ 1		14
Capital contribution from parent	825		$ 825
Capital contribution returned to parent	(20)							(20)
Dividend to parent	(20)			(20)
Other	(1)			(1)
Ending balance at Sep. 30, 2019	4,069		$ 3,685	214		(3)		173
Ending balance (in shares) at Sep. 30, 2019			40
Beginning balance at Jun. 30, 2019	3,876		$ 3,635	72		(4)		173
Beginning balance (in shares) at Jun. 30, 2019			40
Total comprehensive income (loss) available (attributable) to common shareholder	143			142		1
Capital contribution from parent	50		$ 50
Ending balance at Sep. 30, 2019	4,069		$ 3,685	214		(3)		173
Ending balance (in shares) at Sep. 30, 2019			40
Beginning balance at Dec. 31, 2019	3,892		$ 3,695	20		(3)		180
Beginning balance (in shares) at Dec. 31, 2019			40
Total comprehensive income (loss) available (attributable) to common shareholder	266			258				8
Capital contribution from parent	322		$ 322
Ending balance at Sep. 30, 2020	4,480		$ 4,017	278		(3)		188
Ending balance (in shares) at Sep. 30, 2020			40
Beginning balance at Jun. 30, 2020	4,052		$ 3,695	177		(3)		183
Beginning balance (in shares) at Jun. 30, 2020			40
Total comprehensive income (loss) available (attributable) to common shareholder	106			101				5
Capital contribution from parent	322		$ 322
Ending balance at Sep. 30, 2020	$ 4,480		$ 4,017	$ 278		$ (3)		$ 188
Ending balance (in shares) at Sep. 30, 2020			40