Short-Term Borrowings and Bank Lines of Credit (Details) - USD ($)	Jan. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Short-term Debt [Line Items]
Cash and Cash Equivalents, at Carrying Value		$ 589,000,000	$ 199,000,000	$ 131,000,000	$ 85,000,000
Subsequent Event
Short-term Debt [Line Items]
Maximum amount borrowed under revolving credit facility	$ 5,000,000,000
Available Liquidity	4,740,000,000
Cash, Available Liquidity	358,000,000
Total Available Liquidity	5,098,000,000
FirstEnergy | Line of Credit | Subsequent Event
Short-term Debt [Line Items]
Available Liquidity	3,740,000,000
FET | Line of Credit | Subsequent Event
Short-term Debt [Line Items]
Available Liquidity	1,000,000,000
FES
Short-term Debt [Line Items]
Cash and Cash Equivalents, at Carrying Value		1,000,000	2,000,000	$ 2,000,000	$ 2,000,000
FES | Subsequent Event
Short-term Debt [Line Items]
Maximum amount borrowed under revolving credit facility	500,000,000
Total Available Liquidity	501,000,000
FES | Line of Credit | Subsequent Event
Short-term Debt [Line Items]
Available Liquidity	500,000,000
Revolving Credit Facility | Line of Credit
Short-term Debt [Line Items]
Maximum amount borrowed under revolving credit facility		$ 5,000,000,000	500,000,000
Revolving Credit Facility | Line of Credit | Subsequent Event
Short-term Debt [Line Items]
Maximum amount borrowed under revolving credit facility	5,000,000,000
Revolving Credit Facility | FirstEnergy | Line of Credit | Subsequent Event
Short-term Debt [Line Items]
Maximum amount borrowed under revolving credit facility	4,000,000,000
Revolving Credit Facility | FET | Line of Credit | Subsequent Event
Short-term Debt [Line Items]
Maximum amount borrowed under revolving credit facility	1,000,000,000
Revolving Credit Facility | FES | Line of Credit
Short-term Debt [Line Items]
Maximum amount borrowed under revolving credit facility			$ 500,000,000
Revolving Credit Facility | FES | Line of Credit | Subsequent Event
Short-term Debt [Line Items]
Maximum amount borrowed under revolving credit facility	$ 500,000,000