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SPP OATT Upgrade Costs (Details) $ in Millions | 6 Months Ended
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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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