ACCUMULATED OTHER COMPREHENSIVE INCOME- Components of AOCI (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning Balance	$ 6,975	$ 6,814	$ 3,925
Other comprehensive income before reclassifications		(2)	(106)
Amounts reclassified from AOCI	(33)	(27)	(67)
Deconsolidation of FES and FENOC			5
Other comprehensive loss	(33)	(29)	(168)
Income tax benefits on other comprehensive loss	(8)	(8)	(67)
Other comprehensive loss, net of tax	(25)	(21)	(101)
Ending Balance	7,237	6,975	6,814
Accumulated Other Comprehensive Income
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning Balance	20	41	142
Other comprehensive loss, net of tax	(25)	(21)	(101)
Ending Balance	(5)	20	41
Gains & Losses on Cash Flow Hedges
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning Balance	(9)	(11)	(22)
Other comprehensive income before reclassifications		0	0
Amounts reclassified from AOCI	1	2	8
Deconsolidation of FES and FENOC			13
Other comprehensive loss	1	2	21
Income tax benefits on other comprehensive loss	0	0	10
Other comprehensive loss, net of tax	1	2	11
Ending Balance	(8)	(9)	(11)
Unrealized Gains on AFS Securities
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning Balance	0	0	67
Other comprehensive income before reclassifications		0	(97)
Amounts reclassified from AOCI	0	0	(1)
Deconsolidation of FES and FENOC			(8)
Other comprehensive loss	0	0	(106)
Income tax benefits on other comprehensive loss	0	0	(39)
Other comprehensive loss, net of tax	0	0	(67)
Ending Balance	0	0	0
Defined Benefit Pension & OPEB Plans
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning Balance	29	52	97
Other comprehensive income before reclassifications		(2)	(9)
Amounts reclassified from AOCI	(34)	(29)	(74)
Deconsolidation of FES and FENOC			0
Other comprehensive loss	(34)	(31)	(83)
Income tax benefits on other comprehensive loss	(8)	(8)	(38)
Other comprehensive loss, net of tax	(26)	(23)	(45)
Ending Balance	$ 3	$ 29	$ 52