Quarterly Financial Data (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Interim Period, Costs Not Allocable [Line Items]
Operating Revenues	$ 1,122	$ 1,320	$ 948	$ 1,091	$ 1,353	$ 1,192	$ 1,090	$ 946	$ 4,481	$ 4,581	$ 4,495
Operating income (loss)	94	(346)	35	178	271	189	16	(79)	(39)	397	(293)
Income (Loss) from continuing operations after income taxes	(62)	(401)	26	96	149	94	2	(58)	(341)	187	(261)
Income (Loss) from Discontinued Operations (net of income taxes)	0	0	0	0	213	7	11	(8)	0	223	32
Net Income (Loss) Attributable to Parent	$ (62)	$ (401)	$ 26	$ 96	$ 362	$ 101	$ 13	$ (66)	$ (341)	$ 410	$ (230)
Basic (in dollars per share)	$ (0.48)	$ (3.12)	$ 0.26	$ 1.15	$ 1.78	$ 1.13	$ 0.03	$ (0.69)	$ (3.10)	$ 2.24	$ (3.13)
Diluted (in dollars per share)	(0.48)	(3.12)	0.26	1.15	1.78	1.13	0.03	(0.69)	(3.10)	2.24	(3.13)
Basic (in dollars per share)	(0.48)	(3.12)	0.26	1.15	4.33	1.21	0.16	(0.79)	(3.10)	4.91	(2.75)
Diluted (in dollars per share)	$ (0.48)	$ (3.12)	$ 0.26	$ 1.15	$ 4.33	$ 1.21	$ 0.16	$ (0.79)	$ (3.10)	$ 4.91	$ (2.75)
Talen Montana
Interim Period, Costs Not Allocable [Line Items]
Gain					$ 137
Previously Reported
Interim Period, Costs Not Allocable [Line Items]
Operating Revenues		$ 1,419	$ 1,065	$ 946	2,083	$ 1,601	$ 1,007	$ (955)
Operating income (loss)		(246)	34
Income (Loss) from continuing operations after income taxes		(339)	25
Income (Loss) from Discontinued Operations (net of income taxes)		(62)	1
Scenario, Adjustment
Interim Period, Costs Not Allocable [Line Items]
Operating Revenues		(135)	(125)	$ 145	(730)	(409)	83	1,901
Scenario, Adjustment | Sapphire assets
Interim Period, Costs Not Allocable [Line Items]
Operating Revenues		36	8	$ 0	$ 0	$ 0	$ 0	$ 0
Operating income (loss)		(100)	1
Income (Loss) from continuing operations after income taxes		(62)	1
Income (Loss) from Discontinued Operations (net of income taxes)		$ 62	$ (1)