SPP OATT Upgrade Costs (Details) $ in Millions	6 Months Ended
Jun. 30, 2018 USD ($)
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

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