Assets and Liabilities with Recurring Fair Value Measurements Quantitative Information about Level 3 Fair Value Measurements (Details) - USD ($)		Dec. 31, 2019	Dec. 31, 2018
Quantitative Information about Level 3 fair Value Measurements [Line Items]
Derivative, Fair Value, Net	[1]	$ 114,000,000	$ (141,000,000)
Physical Power [Member]
Quantitative Information about Level 3 fair Value Measurements [Line Items]
Derivative, Fair Value, Net	[2]	158,000,000	36,000,000
Physical Power [Member] | Minimum [Member]
Quantitative Information about Level 3 fair Value Measurements [Line Items]
Fair Value Inputs Quantitative Information		4.85	2.12
Physical Power [Member] | Maximum [Member]
Quantitative Information about Level 3 fair Value Measurements [Line Items]
Fair Value Inputs Quantitative Information		184.15	227.98
Natural Gas [Member]
Quantitative Information about Level 3 fair Value Measurements [Line Items]
Derivative, Fair Value, Net		(20,000,000)	(73,000,000)
Natural Gas [Member] | Minimum [Member]
Quantitative Information about Level 3 fair Value Measurements [Line Items]
Fair Value Inputs Quantitative Information		1.73	0.75
Natural Gas [Member] | Maximum [Member]
Quantitative Information about Level 3 fair Value Measurements [Line Items]
Fair Value Inputs Quantitative Information		6.45	8.87
Power Congestion Products [Member]
Quantitative Information about Level 3 fair Value Measurements [Line Items]
Derivative, Fair Value, Net		17,000,000	26,000,000
Power Congestion Products [Member] | Minimum [Member]
Quantitative Information about Level 3 fair Value Measurements [Line Items]
Fair Value Inputs Quantitative Information		(10.32)	(11.71)
Power Congestion Products [Member] | Maximum [Member]
Quantitative Information about Level 3 fair Value Measurements [Line Items]
Fair Value Inputs Quantitative Information		$ 20.00	$ 11.88

[1] At December 31, 2019 and 2018, we had $191 million and $244 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.

[2] Power contracts include power and heat rate instruments classified as level 3 in the fair value hierarchy.