Commitments and Contingencies SPP Filing to Assign GridLiance Facilities to SPS Rate Zone (Details) - Southwest Power Pool (SPP) [Member] - SPP Open Access Transmission Tariff Upgrade Costs [Member] - USD ($) $ in Millions	1 Months Ended		12 Months Ended
	Mar. 16, 2020	Aug. 31, 2018	Dec. 31, 2020
Public Utilities, General Disclosures [Line Items]
Public Utilities, Annual Transmission Revenue Requirement Increase (SPS)	$ 9	$ 6
Public Utilities, Annual Transmission Revenue Requirement Increase	$ 14	10
Public Utilities, Annual Transmission Revenue Requirement Increase, Other Utilities		$ 4
SPS [Member]
Public Utilities, General Disclosures [Line Items]
Public Utilities, Annual Transmission Revenue Requirement Increase (SPS)			$ 15