Fair Value Measurements (Schedule of Fair Value of the Level 3 Assets and Liabilities by Major Contract Type (All Related to Commodity Contracts) and the Significant Unobservable Inputs Used in the Valuations) (Details) - Derivative financial instruments, assets and liabilities [Member] - Level 3 [Member] $ in Millions		9 Months Ended	12 Months Ended
		Sep. 30, 2015 USD ($) $ / Megawatt-hour	Dec. 31, 2014 USD ($) $ / T $ / Megawatt-hour
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Assets	[1]	$ 34	$ 49
Liabilities	[1]	(7)	(14)
Total	[1]	27	35
Electricity purchases and sales [Member] | Valuation Model [Member]
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Assets	[1]	2	4
Liabilities	[1]	(1)	(5)
Total	[1]	1	(1)
Electricity congestion revenue rights [Member] | Market Approach [Member]
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Assets	[1],[2],[3]	27	38
Liabilities	[1],[2],[3]	(3)	(4)
Total	[1],[2],[3]	24	34
Coal purchases [Member] | Market Approach [Member]
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Assets	[1],[2]		0
Liabilities	[1],[2]		(4)
Total	[1],[2]		(4)
Other [Member]
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Assets	[1],[4]	5	7
Liabilities	[1],[4]	(3)	(1)
Total	[1],[4]	$ 2	$ 6
Minimum [Member] | Electricity purchases and sales [Member] | Valuation Model [Member]
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Illiquid pricing locations (in usd per MWh) | $ / Megawatt-hour	[1],[5],[6]	25	30
Hourly price curve shape (in usd per MWh) | $ / Megawatt-hour	[1],[6],[7]	15	20
Minimum [Member] | Electricity congestion revenue rights [Member] | Market Approach [Member]
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Illiquid price differences between settlement points | $ / Megawatt-hour	[1],[3],[6],[8]	0	0.00
Minimum [Member] | Coal purchases [Member] | Market Approach [Member]
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Illiquid price variances between mines | $ / T	[1],[2],[6]		0.00
Fair Value Inputs, Illiquid Pricing Variances Between Heat Content | $ / T	[9]		0.00
Maximum [Member] | Electricity purchases and sales [Member] | Valuation Model [Member]
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Illiquid pricing locations (in usd per MWh) | $ / Megawatt-hour	[1],[5],[6]	40	50
Hourly price curve shape (in usd per MWh) | $ / Megawatt-hour	[1],[6],[7]	55	70
Maximum [Member] | Electricity congestion revenue rights [Member] | Market Approach [Member]
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Illiquid price differences between settlement points | $ / Megawatt-hour	[1],[3],[6],[8]	10	20.00
Maximum [Member] | Coal purchases [Member] | Market Approach [Member]
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Illiquid price variances between mines | $ / T	[1],[2],[6]		1.00
Fair Value Inputs, Illiquid Pricing Variances Between Heat Content | $ / T	[9]		1.00

[1] Electricity purchase and sales contracts include hedging positions in the ERCOT regions, as well as power contracts, the valuations of which include unobservable inputs related to the hourly shaping of the price curve. Electricity congestion revenue rights contracts consist of forward purchase contracts (swaps and options) used to hedge electricity price differences between settlement points within ERCOT. Coal purchase contracts relate to western (Powder River Basin) coal.

[2] Based on the historical range of price variances between mine locations.

[3] While we use the market approach, there is either insufficient market data to consider the valuation liquid or the significance of credit reserves or non-performance risk adjustments results in a Level 3 designation.

[4] Other includes contracts for ancillary services, natural gas, power options, diesel options and coal options.

[5] Based on the historical range of forward average monthly ERCOT hub and load zone prices.

[6] The range of the inputs may be influenced by factors such as time of day, delivery period, season and location.

[7] Based on the historical range of forward average hourly ERCOT North Hub prices.

[8] Based on the historical price differences between settlement points within the ERCOT hubs and load zones.

[9] Based on historical ranges of forward average prices between different heat contents (potential energy in coal for a given mass).