Commitments and Contingencies SPP OATT Upgrade Costs (Details) $ in Millions	12 Months Ended
Dec. 31, 2017 USD ($)
SPS \| Southwest Power Pool (SPP) [Member] \| SPP Open Access Transmission Tariff Upgrade Costs [Member]
Loss Contingencies [Line Items]
Public Utilities, Billed Charges For Transmission Service Upgrades	$ 13

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