Assets and Liabilities with Recurring Fair Value Measurements Quantitative Information about Level 3 Fair Value Measurements (Details) - USD ($)		Dec. 31, 2018	Dec. 31, 2017
Quantitative Information about Level 3 fair Value Measurements [Line Items]
Derivative, Fair Value, Net	[1]	$ (141,000,000)	$ 76,000,000
Physical Power [Member]
Quantitative Information about Level 3 fair Value Measurements [Line Items]
Derivative, Fair Value, Net		36,000,000	149,000,000
Physical Power [Member] | Minimum [Member]
Quantitative Information about Level 3 fair Value Measurements [Line Items]
Fair Value Inputs Quantitative Information		2.12	4.13
Physical Power [Member] | Maximum [Member]
Quantitative Information about Level 3 fair Value Measurements [Line Items]
Fair Value Inputs Quantitative Information		227.98	119.20
Natural Gas [Member]
Quantitative Information about Level 3 fair Value Measurements [Line Items]
Derivative, Fair Value, Net		(73,000,000)	34,000,000
Natural Gas [Member] | Minimum [Member]
Quantitative Information about Level 3 fair Value Measurements [Line Items]
Fair Value Inputs Quantitative Information		0.75	1.62
Natural Gas [Member] | Maximum [Member]
Quantitative Information about Level 3 fair Value Measurements [Line Items]
Fair Value Inputs Quantitative Information		8.87	13.67
Power Congestion Products [Member]
Quantitative Information about Level 3 fair Value Measurements [Line Items]
Derivative, Fair Value, Net		26,000,000	11,000,000
Power Congestion Products [Member] | Minimum [Member]
Quantitative Information about Level 3 fair Value Measurements [Line Items]
Fair Value Inputs Quantitative Information		(11.71)	(10.54)
Power Congestion Products [Member] | Maximum [Member]
Quantitative Information about Level 3 fair Value Measurements [Line Items]
Fair Value Inputs Quantitative Information		$ 11.88	$ 9.13

[1] At December 31, 2018 and 2017, we had $244 million and $155 million of collateral under master netting arrangements that were not offset against our derivative instruments on the Consolidated Balance Sheets primarily related to initial margin requirements.