Commitments and Contingencies (Operating Leases) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Coal, Gas and Transportation Commitments
Off-market Lease, Unfavorable		$ 1,040	$ 1,146
Operating Leases, Future Minimum Payments Due, Future Minimum Sublease Rentals		$ 14
EME [Member]
Coal, Gas and Transportation Commitments
Leased Interest		100.00%
Off-market Lease, Unfavorable				$ 159
Lease expense		$ 14
GenOn Mid-Atlantic
Coal, Gas and Transportation Commitments
Leased Interest		100.00%
Off-market Lease, Unfavorable					$ 604
Lease expense		$ 43
Future commitments under coal, gas and transportation contractual agreements
Operating Leases, Future Minimum Payments Due, Next Twelve Months		144
Operating Leases, Future Minimum Payments, Due in Two Years		105
Operating Leases, Future Minimum Payments, Due in Three Years		139
Operating Leases, Future Minimum Payments, Due in Four Years		105
Operating Leases, Future Minimum Payments, Due in Five Years		42
Thereafter		400
Total		935
REMA [Member]
Coal, Gas and Transportation Commitments
Lease expense		29
Future commitments under coal, gas and transportation contractual agreements
Operating Leases, Future Minimum Payments Due, Next Twelve Months		63
Operating Leases, Future Minimum Payments, Due in Two Years		55
Operating Leases, Future Minimum Payments, Due in Three Years		65
Operating Leases, Future Minimum Payments, Due in Four Years		56
Operating Leases, Future Minimum Payments, Due in Five Years		47
Thereafter		231
Total		517
Other Leased Property [Member]
Coal, Gas and Transportation Commitments
Lease expense		102	$ 100	$ 106
Future commitments under coal, gas and transportation contractual agreements
Operating Leases, Future Minimum Payments Due, Next Twelve Months		84
Operating Leases, Future Minimum Payments, Due in Two Years		76
Operating Leases, Future Minimum Payments, Due in Three Years		67
Operating Leases, Future Minimum Payments, Due in Four Years		61
Operating Leases, Future Minimum Payments, Due in Five Years		52
Thereafter		443
Total	[1]	783
Powerton and Joliet [Member]
Future commitments under coal, gas and transportation contractual agreements
Operating Leases, Future Minimum Payments Due, Next Twelve Months		1
Operating Leases, Future Minimum Payments, Due in Two Years		1
Operating Leases, Future Minimum Payments, Due in Three Years		1
Operating Leases, Future Minimum Payments, Due in Four Years		1
Operating Leases, Future Minimum Payments, Due in Five Years		3
Thereafter		234
Total		$ 241
REMA [Member]
Coal, Gas and Transportation Commitments
Leased Interest		100.00%
REMA [Member] | Keystone [Member]
Coal, Gas and Transportation Commitments
Leased Interest		16.70%
REMA [Member] | Conemaugh [Member]
Coal, Gas and Transportation Commitments
Leased Interest		16.50%
REMA [Member] | Keystone Conemaugh
Coal, Gas and Transportation Commitments
Off-market Lease, Unfavorable					$ 186

[1] Amounts in the table exclude future sublease income of $14 million associated with long-term leases for office locations.