REGULATORY MATTERS - Reliability and FERC Matters (Details) - USD ($) $ in Millions			12 Months Ended
	Feb. 02, 2021	Nov. 01, 2020	Dec. 31, 2020	Dec. 31, 2019
ATSI | FERC
Regulatory Matters [Line Items]
Regulatory asset balance			$ 79	$ 73
ATSI | Regulated Transmission
Regulatory Matters [Line Items]
Approved ROE			10.38%
MAIT | Regulated Transmission
Regulatory Matters [Line Items]
Allowed Debt			60.00%
Approved ROE			10.30%
TrAIL | Regulated Transmission | TrAIL the Line and Black Oak SVC
Regulatory Matters [Line Items]
Approved ROE			12.70%
TrAIL | Regulated Transmission | All Other Projects
Regulatory Matters [Line Items]
Approved ROE			11.70%
JCP&L
Regulatory Matters [Line Items]
Allowed Debt		48.56%	55.00%
Approved ROE		9.60%	9.60%
JCP&L | Regulated Transmission
Regulatory Matters [Line Items]
Approved ROE			10.80%
JCP&L | Regulated Transmission | Subsequent Event
Regulatory Matters [Line Items]
Approved ROE	10.20%