Summary of Significant Accounting Policies (GenOn, GenOn Americas Generation and GenOn Mid-Atlantic) (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Condensed Balance Sheet Statements, Captions [Line Items]
Property, plant and equipment Accumulated depreciation	$ 730	$ 604
Intangible assets Accumulated amortization	66	87
GenOn Americas Generation
Condensed Balance Sheet Statements, Captions [Line Items]
Property, plant and equipment Accumulated depreciation	329	275
Intangible assets Accumulated amortization	66	87
GenOn Mid-Atlantic
Condensed Balance Sheet Statements, Captions [Line Items]
Property, plant and equipment Accumulated depreciation	284	237
Intangible assets Accumulated amortization	$ 33	$ 29