Assets and Liabilities with Recurring Fair Value Measurements Quantitative Info on Level 3 (Details) - USD ($)		Mar. 31, 2018	Dec. 31, 2017
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Derivative, Fair Value, Net	[1]	$ (91,000,000)	$ 76,000,000
Power Contracts [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Derivative, Fair Value, Net		94,000,000	149,000,000
Power Contracts [Member] | Minimum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs Quantitative Information		2.58	4.13
Power Contracts [Member] | Maximum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs Quantitative Information		215.31	119.20
Natural Gas [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Derivative, Fair Value, Net		11,000,000	34,000,000
Natural Gas [Member] | Minimum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs Quantitative Information		0.95	1.62
Natural Gas [Member] | Maximum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs Quantitative Information		10.05	13.67
Power Congestion Products [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Derivative, Fair Value, Net		4,000,000	11,000,000
Power Congestion Products [Member] | Minimum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs Quantitative Information		(7.52)	(10.54)
Power Congestion Products [Member] | Maximum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs Quantitative Information		$ 9.40	$ 9.13

[1] At March 31, 2018 and December 31, 2017, we had $206 million and $155 million of collateral under master netting arrangements that were not offset against our derivative instruments on the Consolidated Condensed Balance Sheets primarily related to initial margin requirements.