Commitments and Contingencies, Legal Contingencies (Details) - USD ($) $ in Millions	1 Months Ended	12 Months Ended
	Aug. 31, 2018	Dec. 31, 2018	Dec. 31, 2016
Loss Contingencies [Line Items]
Public Utilities, Amount proposed changes would increase wholesale transmission revenues		$ 9.4
Legal Contingencies [Abstract]
Public Utilities, Amount of proposed changes would be recoverable		$ 4.4
Southwest Power Pool (SPP) [Member] | SPP Open Access Transmission Tariff Upgrade Costs [Member]
Loss Contingencies [Line Items]
Public Utilities, Annual Transmission Revenue Requirement Increase	$ 9.5
Legal Contingencies [Abstract]
Public Utilities, Annual Transmission Revenue Requirement Increase (SPS)	6.0
Public Utilities, Annual Transmission Revenue Requirement (Other Utilities)	$ 3.5
SPS [Member] | Southwest Power Pool (SPP) [Member] | SPP Open Access Transmission Tariff Upgrade Costs [Member]
Loss Contingencies [Line Items]
Public Utilities, Billed Charges For Transmission Service Upgrades			$ 13.0