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Commitments and Contingencies - SPP OATT Upgrade Costs (Details)
$ in Millions
6 Months Ended
Jun. 30, 2018
USD ($)
Southwest Power Pool (SPP) | SPP Open Access Transmission Tariff Upgrade Costs | SPS  
Public Utilities, General Disclosures [Line Items]  
Charges For Transmission Service Upgrades $ 13