Commitments and Contingencies - Nuclear Spent Fuel and Waste Disposal (Details) - PNM - Nuclear spent fuel and waste disposal - Palo Verde Nuclear Generating Station - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Public Utilities, Commitments And Contingencies [Line Items]
Estimate of possible loss	$ 57.7
Other deferred credits
Public Utilities, Commitments And Contingencies [Line Items]
Loss contingency accrual	$ 12.4	$ 12.3

Reflects the estimated amount of loss from the specified contingency as of the balance sheet date.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section 50
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Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
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Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 460
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-Section 50
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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant