Commitments and Contingencies - SPP OATT Upgrade Costs (Details) $ in Millions	3 Months Ended
Mar. 31, 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

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