Segment Information - Major Customers (Details) - Customer Concentration Risk - Texas-New Mexico Power Company - Electric operating revenues	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
REP A
Concentration Risk [Line Items]
Operating revenues from continuing operations	22.00%	21.00%	16.00%
REP B
Concentration Risk [Line Items]
Operating revenues from continuing operations	17.00%	15.00%	11.00%
REP C
Concentration Risk [Line Items]
Operating revenues from continuing operations	12.00%	12.00%	10.00%

For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

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