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 $ in Millions	1 Months Ended
	Aug. 31, 2018 USD ($)
Public Utilities, General Disclosures [Line Items]
Annual Transmission Revenue Requirement Increase	$ 9.5
Annual Transmission Revenue Requirement (Other Utilities)	3.5
SPS
Public Utilities, General Disclosures [Line Items]
Annual Transmission Revenue Requirement Increase (SPS)	$ 6.0