Long-term Debt and Finance Leases - Tax Exempt Bonds (Details) - USD ($)	Dec. 31, 2021	Dec. 31, 2020	Dec. 17, 2020	Mar. 11, 2020
Debt Instrument [Line Items]
Long-term debt	$ 8,100,000,000	$ 8,855,000,000
NRG Indian River Power 2020, tax exempt bonds, due 2040
Debt Instrument [Line Items]
Long-term debt	$ 57,000,000	57,000,000
Interest rate, stated percentage	1.25%		1.25%
Aggregate principal amount			$ 57,000,000
NRG Indian River Power 2020, tax exempt bonds, due 2045
Debt Instrument [Line Items]
Long-term debt	$ 190,000,000	190,000,000
Interest rate, stated percentage	1.25%		1.25%
Aggregate principal amount			$ 190,000,000
NRG Dunkirk 2020, tax exempt bonds, due 2042
Debt Instrument [Line Items]
Long-term debt	$ 59,000,000	59,000,000
Interest rate, stated percentage	1.30%			1.30%
Aggregate principal amount				$ 59,000,000
City of Texas City, tax exempt bonds, due 2045
Debt Instrument [Line Items]
Long-term debt	$ 33,000,000	33,000,000
Interest rate, stated percentage	4.125%
Fort Bend County, tax exempt bonds, due 2038
Debt Instrument [Line Items]
Long-term debt	$ 54,000,000	54,000,000
Interest rate, stated percentage	4.75%
Fort Bend County, tax exempt bonds, due 2042
Debt Instrument [Line Items]
Long-term debt	$ 73,000,000	73,000,000
Interest rate, stated percentage	4.75%
Tax-exempt bonds
Debt Instrument [Line Items]
Long-term debt	$ 466,000,000	$ 466,000,000
5.875% Dunkirk Power LLC Tax Exempt Bonds Due 2042
Debt Instrument [Line Items]
Interest rate, stated percentage	5.875%
5.375% Indian River Power LLC Tax Exempt Bonds Due 2045
Debt Instrument [Line Items]
Interest rate, stated percentage	5.375%
6.000% Indian River Power LLC Tax Exempt Bonds Due 2040
Debt Instrument [Line Items]
Interest rate, stated percentage	6.00%