Commitments and Contingencies, Rate Matters, SPS (Details) - USD ($) $ in Millions	1 Months Ended	6 Months Ended	12 Months Ended
	Aug. 31, 2018	Jun. 30, 2019	Dec. 31, 2018	Dec. 31, 2016
Public Utilities, General Disclosures [Line Items]
Public Utilities, Amount proposed changes would increase wholesale transmission revenues			$ 9.4
Public Utilities, Amount of proposed changes would be recoverable			$ 4.4
Southwest Power Pool (SPP) | SPP Open Access Transmission Tariff Upgrade Costs
Public Utilities, General Disclosures [Line Items]
Public Utilities, Annual Transmission Revenue Requirement Increase	$ 9.5
Public Utilities, Annual Transmission Revenue Requirement Increase (SPS)	6.0
Public Utilities, Annual Transmission Revenue Requirement (Other Utilities)	$ 3.5
FERC [Member] | SPP Open Access Transmission Tariff Upgrade Costs
Public Utilities, General Disclosures [Line Items]
Hearing Procedures Duration		13 months
SPS | Southwest Power Pool (SPP) | SPP Open Access Transmission Tariff Upgrade Costs
Public Utilities, General Disclosures [Line Items]
Public Utilities, Billed Charges For Transmission Service Upgrades				$ 13.0