Financing Activities (Details)		1 Months Ended			9 Months Ended
		Oct. 31, 2015 USD ($)	Sep. 30, 2015 USD ($) shares	May. 31, 2015 USD ($)	Sep. 30, 2015 USD ($) shares	Sep. 30, 2014 USD ($)	Nov. 02, 2015 USD ($)	Aug. 31, 2015	Jul. 31, 2015	Jun. 30, 2015 USD ($)	Jun. 01, 2015 USD ($)
Credit Arrangments and Short-term Debt
Unamortized fees included in interest expense					$ 12,000,000
Long Term Debt
Total aggregate consideration paid to repurchase debt					$ 33,000,000	$ 308,000,000
Preferred Stock
Preferred stock, shares authorized (in shares) | shares			100,000,000		100,000,000
Preferred stock, shares issued (in shares) | shares			0		0
Preferred stock, shares outstanding (in shares) | shares			0		0
RJS Power
Long Term Debt
Long term debt assumed											$ 1,244,000,000
Exempt Facilities Revenue Refunding Bonds
Long Term Debt
Debt instrument, face amount			$ 231,000,000		$ 231,000,000
Interest rate								3.00%
Term Loan and Revolving Credit Facility | MACH Gen. LLC | Subsequent event
Long Term Debt
Debt outstanding							$ 578,000,000
Term Loan | MACH Gen. LLC | Subsequent event
Credit Arrangments and Short-term Debt
Weighted average interest rate							6.21%
Long Term Debt
Debt outstanding							$ 475,000,000
Series 2009A Bonds | Exempt Facilities Revenue Refunding Bonds
Long Term Debt
Debt instrument, face amount			$ 100,000,000		$ 100,000,000
Interest rate			6.40%		6.40%
Series 2009B and 2009C Bonds | Exempt Facilities Revenue Refunding Bonds
Long Term Debt
Debt instrument, face amount			$ 131,000,000		$ 131,000,000
Interest rate			5.00%		5.00%
Fixed interest rate period until mandatory repurchase and optional remarketing			5 years
Talen Energy Supply
Long Term Debt
Total aggregate consideration paid to repurchase debt					$ 33,000,000	$ 308,000,000
Talen Energy Supply | Redeemable Senior Unsecured Notes due 2025 | Unsecured Debt
Long Term Debt
Debt instrument, face amount				$ 600,000,000
Interest rate				6.50%
Proceeds from debt, net				$ 591,000,000
Talen Energy Supply | Senior Notes due 2019 | Senior Notes
Long Term Debt
Interest rate									4.625%
Talen Energy Supply | Senior Notes due 2019 | Senior Notes | RJS Power
Long Term Debt
Interest rate										5.125%
Long term debt assumed										$ 1,250,000,000
Talen Energy Supply | Reset Put Securities due 2035 | REset Put Securities | Subsequent event
Long Term Debt
Debt instrument, face amount		$ 300,000,000
Interest rate		5.70%
Total aggregate consideration paid to repurchase debt		$ 434,000,000
Principal amount repurchased		300,000,000
Remarketing option value paid		134,000,000
Talen Energy Supply | On or prior to June 1, 2020 | Redeemable Senior Unsecured Notes due 2025 | Unsecured Debt
Long Term Debt
Redemption price percentage				100.00%
Talen Energy Supply | On or prior to June 1, 2018 | Redeemable Senior Unsecured Notes due 2025 | Unsecured Debt
Long Term Debt
Redemption price percentage				106.50%
Redemption price, percentage of principal amount that may be redeemed				35.00%
Syndicated Secured Credit Facility
Credit Arrangments and Short-term Debt
Expiration Date	[1]				Jun. 30, 2020
Capacity	[1]		$ 1,850,000,000		$ 1,850,000,000
Borrowed	[1]		0		0
Letters of Credit Issued	[1]		334,000,000		334,000,000
Unused Capacity	[1]		$ 1,516,000,000		$ 1,516,000,000
Covenant, ratio of secured debt to adjusted EBITDA			4.50		4.50
Syndicated Secured Credit Facility | Talen Energy Supply
Credit Arrangments and Short-term Debt
Carrying value of property subject to lien to support facility			$ 7,000,000,000		$ 7,000,000,000
Syndicated Secured Credit Facility | Talen Energy Supply | Subsequent event
Credit Arrangments and Short-term Debt
Borrowed		550,000,000
Amount borrowed under facility		$ 400,000,000
Weighted average interest rate		2.44%
Facility Agreement | Talen Energy Supply
Credit Arrangments and Short-term Debt
Expiration Date					Jun. 30, 2017
Potential maximum Facility Agreement capacity			500,000,000		$ 500,000,000
Secured Trading Facility | Talen Energy Supply
Credit Arrangments and Short-term Debt
Expiration Date					Nov. 03, 2019
Secured energy marketing and trading facility capacity			$ 800,000,000		$ 800,000,000
Facility term					5 years
Revolving Credit Facility | MACH Gen. LLC | Subsequent event
Credit Arrangments and Short-term Debt
Capacity							160,000,000
Borrowed							$ 103,000,000
Weighted average interest rate							4.98%

[1] The facility includes capacity available for letters of credit and for short-term borrowings. The facility requires Talen Energy Supply to maintain a senior secured debt to adjusted EBITDA ratio (as defined in the agreement) of less than or equal to 4.50 to 1.00 as of the last day of any fiscal quarter. The company pays customary fees on the facility and borrowings bear interest at the company's option at either a defined base rate or LIBOR-based rates, in each case plus an applicable margin.