Derivative Instruments and Hedging Activities (Intro) (Details)		3 Months Ended		9 Months Ended
		Sep. 30, 2015 USD ($) MW	Sep. 30, 2014 USD ($)	Sep. 30, 2015 USD ($) MW	Sep. 30, 2014 USD ($)	Dec. 31, 2014 USD ($)
Commodity Price Risk (Non-trading) [Abstract]
Generation fleet capacity (in MW) | MW		15,053		15,053
Cash Flow Hedges
Net unrealized after-tax gains (losses) expected to be reclassified into earnings within 12 months related to commodity price risk cash flow hedge contracts		$ 14,000,000		$ 14,000,000
Commodity Price Risk Nontrading Economic Activity Numeric [Abstract]
Year of expiration of the maximum maturity date of economic activity derivative contracts				2020
Wholesale energy
Pre-tax Gains (Losses) of Economic Activity (Details) [Abstract]
Pre-tax gain (loss)	[1]	(77,000,000)	$ 299,000,000	$ (234,000,000)	$ (581,000,000)
Unrealized gain from change in valuation technique		30,000,000
Retail Energy
Pre-tax Gains (Losses) of Economic Activity (Details) [Abstract]
Pre-tax gain (loss)		(3,000,000)	2,000,000	(25,000,000)	(20,000,000)
Fuel
Pre-tax Gains (Losses) of Economic Activity (Details) [Abstract]
Pre-tax gain (loss)		(3,000,000)	(9,000,000)	13,000,000	(3,000,000)
Energy purchases
Pre-tax Gains (Losses) of Economic Activity (Details) [Abstract]
Pre-tax gain (loss)	[1]	130,000,000	$ (217,000,000)	370,000,000	402,000,000
Credit risk
Master Netting Arrangements
Obligation to return counterparty cash collateral under master netting arrangements		7,000,000		7,000,000		$ 11,000,000
Cash collateral posted under master netting arrangements		$ 0		$ 0		$ 0
Price Risk Derivative
Pre-tax Gains (Losses) of Economic Activity (Details) [Abstract]
Net energy trading margins					$ 58,000,000

[1] In the third quarter of 2015, Talen Energy refined an input used in its valuation technique for certain PJM basis curves as observable inputs became available. This change resulted in the recording of a $30 million net unrealized gain, primarily reflected in "Wholesale energy" revenue on the Statement of Income.