Electricity Purchase Agreements - Summary of Significant Terms of Electricity Purchase Agreements (Details)	12 Months Ended
	Dec. 31, 2017 MW
Brooklyn Navy Yard
Long-term Contract for Purchase of Electric Power [Line Items]
Plant Output (MW)	322
Contracted Output (MW)	303
Contract Term (Years)	40 years
Indian Point
Long-term Contract for Purchase of Electric Power [Line Items]
Plant Output (MW)	2,150
Contracted Output (MW)	500
Contract Term (Years)	16 years