Summary of Significant Accounting Policies (Details) (USD $) In Millions, unless otherwise specified																			12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2013 Freestone Energy Center [Member]	Dec. 31, 2013 Hidalgo Energy Center [Member]	Dec. 31, 2013 Debt Service		Dec. 31, 2012 Debt Service		Dec. 31, 2013 Rent Reserve [Member]	Dec. 31, 2012 Rent Reserve [Member]	Dec. 31, 2013 Construction Major Maintenance	Dec. 31, 2012 Construction Major Maintenance	Dec. 31, 2013 Security Project Insurance	Dec. 31, 2012 Security Project Insurance	Dec. 31, 2013 Other	Dec. 31, 2012 Other	Dec. 31, 2013 Greenfield [Member]	Dec. 31, 2013 Whitby [Member]	Dec. 31, 2012 Comprehensive Income [Member]	Dec. 31, 2011 Comprehensive Income [Member]	Dec. 31, 2012 Cash Flow Statement [Member]	Dec. 31, 2011 Cash Flow Statement [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Prior Period Reclassification Adjustment																			$ (6)	$ (14)	$ 5	$ 0
Held-to-maturity Securities, Restricted	24	25
Current	203	193			11	[1]	11	[1]	3	0	35	32	151	101	3	49
Non-current	69	60			41	[1]	41	[1]	0	0	20	14	6	3	2	2
Total	272	253			52	[1]	52	[1]	3	0	55	46	157	104	5	51
Prior Period Adjustment [Abstract]
Ownership percentage in equity method investment																	50.00%	50.00%
Cash and cash equivalents subject to project finance facilities and lease agreements	292	131
Inventory	364	301
Property, plant and equipment, salvage value (as a percent)	10.00%
Property, plant and equipment, salvage value of rotables (as a percent)	0.15%
Asset retirement obligations	44	38
Jointly Owned Plants [Abstract]
Jointly Owned Utility Plant, Proportionate Ownership Share			75.00%	78.50%
Jointly Owned Utility Plant, Gross Ownership Amount of Plant in Service			393	255
Jointly Owned Utility Plant, Ownership Amount of Plant Accumulated Depreciation			(135)	(93)
Jointly Owned Utility Plant, Ownership Amount of Construction Work in Progress			$ 0	$ 0

[1] At December 31, 2013 and 2012, amounts restricted for debt service included approximately $24 million and $25 million, respectively, of repurchase agreements with a financial institution containing maturity dates greater than one year.