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Segment Information - Major Customers (Details) - Customer Concentration Risk - Texas-New Mexico Power Company - Electric operating revenues	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
REP A
Concentration Risk [Line Items]
Operating revenues from continuing operations	21.00%	22.00%	21.00%
REP B
Concentration Risk [Line Items]
Operating revenues from continuing operations	18.00%	17.00%	15.00%
REP C
Concentration Risk [Line Items]
Operating revenues from continuing operations	11.00%	12.00%	12.00%

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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