Acquisition (Details) (USD $)	6 Months Ended														3 Months Ended
	Jun. 30, 2014 MW	Dec. 31, 2013 MW	Jun. 30, 2014 Guadalupe Energy Center [Member] MW	Jun. 30, 2014 Guadalupe Expansion Capacity [Member] MW	Jun. 30, 2014 Incremental CCFC Term Loans [Member]	Jun. 30, 2014 Oneta Energy Center [Member] MW	Jun. 30, 2014 Carville Energy Center [Member] MW		Jun. 30, 2014 Decatur Energy Center [Member] MW	Jun. 30, 2014 Hog Bayou Energy Center [Member] MW	Jun. 30, 2014 Santa Rosa Energy Center [Member] MW	Jun. 30, 2014 Columbia Energy Center [Member] MW		Jun. 30, 2014 Six Power Plants [Member] MW	Sep. 30, 2014 Subsequent Event [Member]
Business Acquisition [Line Items]
Other Assets Held-for-sale, Current	$ 26,000,000
Current assets held for sale	808,000,000	0
Number of power plants to be disposed of	6
Ownership percentage before divestiture of business	100.00%
Proceeds from Sale of Productive Assets															1,570,000,000
Working Capital Adjustment to Sale price															4,000,000
Business Combination, Contingent Consideration, Liability															16,000,000
Power generation capacity	10,365	9,427	1,000	400		1,134	501	[1]	795	237	225	606	[1]	3,498
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net			625,000,000	15,000,000
Number of 525 MW generation blocks to be acquired in the Guadalupe acquisition	2
Debt Instrument, Face Amount					425,000,000
Gain (Loss) on Disposition of Property Plant Equipment, Excluding Oil and Gas Property and Timber Property															750,000,000
Assets Held-for-sale, Property, Plant and Equipment	769,000,000
Assets Held-for-sale, Other, Noncurrent	13,000,000
Current liabilities held for sale	$ 9,000,000	$ 0

[1] Indicates combined-cycle cogeneration power plant.