Commitments and Contingencies - SPP Filing to Assign GridLiance Facilities to SPS Rate Zone (Details) - Southwest Power Pool (SPP) - SPP Open Access Transmission Tariff Upgrade Costs - USD ($) $ in Millions	1 Months Ended	3 Months Ended
	Aug. 31, 2018	Mar. 31, 2021	Mar. 16, 2020
Public Utilities, General Disclosures [Line Items]
Annual Transmission Revenue Requirement Increase	$ 10.0		$ 14.0
Annual Transmission Revenue Requirement Increase (SPS)	6.0	$ 17.0	$ 9.0
Annual Transmission Revenue Requirement (Other Utilities)	$ 4.0