Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

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Goodwill (Details) - USD ($)		12 Months Ended
Goodwill (Details) - USD ($)	Apr. 01, 2017	Dec. 31, 2017	Jun. 30, 2018	Apr. 01, 2018	Mar. 31, 2017
Goodwill [Line Items]
Goodwill		$ 278,297,000	$ 278,297,000		$ 278,297,000
Impairments of goodwill	$ 0	0
PNM
Goodwill [Line Items]
Goodwill		51,632,000	51,632,000	$ 51,600,000
Goodwill fair value exceeded by its carrying value				19.00%
TNMP
Goodwill [Line Items]
Goodwill		$ 226,665,000	$ 226,665,000	$ 226,700,000
Goodwill fair value exceeded by its carrying value				32.00%