Regulatory and Rate Matters - New Mexico General Rate Cases (Details) $ in Millions														9 Months Ended	12 Months Ended		30 Months Ended
	Jan. 31, 2018 USD ($)	Jan. 16, 2018 USD ($)	Jan. 10, 2018 USD ($)	Dec. 31, 2017 USD ($)	May 23, 2017 USD ($)	Dec. 07, 2016 USD ($)	Sep. 30, 2016 USD ($)	Sep. 28, 2016 USD ($) MW	Aug. 04, 2016 USD ($) MW	Jan. 15, 2016 USD ($) lease MW	Aug. 27, 2015 USD ($)	Jan. 15, 2015	Dec. 11, 2014 USD ($)	Sep. 30, 2016 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2018 USD ($)	Oct. 31, 2017 USD ($)	Jul. 01, 2016 USD ($)	May 31, 2016 MW
Subsequent Event
Public Utilities, General Disclosures [Line Items]
Increase (decrease) in non-fuel revenue requirement	$ 10.3
Percent of non-fuel revenue requirement change implemented	50.00%
NMPRC
Public Utilities, General Disclosures [Line Items]
Requested rate increase (decrease)						$ 99.2
Requested return on equity						10.125%
Pre-tax regulatory disallowance				$ 27.9												$ 27.9
Public Service Company of New Mexico | NMPRC
Public Utilities, General Disclosures [Line Items]
Requested rate increase (decrease)					$ 62.3
Requested return on equity					9.575%
Proposed term for providing benefits to customers related to reduction in state corporate tax					3 years
Initial rate increase (decrease) amount					$ 32.3
Public Service Company of New Mexico | NMPRC | Subsequent Event
Public Utilities, General Disclosures [Line Items]
Requested rate increase (decrease)		$ (4.4)	$ (9.1)
Requested rate increase (decrease), duplicative amount		$ 4.7
Public Service Company of New Mexico | Clean Air Act, Balanced Draft Technology | San Juan Generating Station Units 1 and 4
Public Utilities, General Disclosures [Line Items]
Net book value				49.4												49.4
Public Service Company of New Mexico | Clean Air Act, Balanced Draft Technology | San Juan Generating Station Units 1 and 4 | Installation costs including construction management, gross receipts taxes, AFUDC, and other PNM costs
Public Utilities, General Disclosures [Line Items]
Portion of costs for SNCRs and BDT equipment									$ 52.3
Public Service Company of New Mexico | Palo Verde Nuclear Generating Station, Unit 2 Leases
Public Utilities, General Disclosures [Line Items]
Number of megawatts purchased (in megawatts) | MW										64.1										64.1
Number of leases under which assets were purchased | lease										3
Estimated annual property tax expense									$ 0.8
Number of leases under which lease term was extended | lease										1
Lease term extension period										8 years
Number of megawatts | MW								114.6	114.6
Net book value				75.3												75.3
Net book value of capital improvements				39.1												39.1
Public Service Company of New Mexico | Palo Verde Nuclear Generating Station, Unit 1 Leases, extended
Public Utilities, General Disclosures [Line Items]
Estimated annual property tax expense										$ 1.5
Estimated annual rent expense									$ 18.1
Number of leases under which lease term was extended | lease										4
Lease term extension period												8 years
Public Service Company of New Mexico | 2014 Electric Rate Case
Public Utilities, General Disclosures [Line Items]
Requested rate increase (decrease)													$ 107.4
Public Service Company of New Mexico | 2015 Electric Rate Case
Public Utilities, General Disclosures [Line Items]
Requested rate increase (decrease)											$ 123.5
Requested return on equity											10.50%
Requested rate increase (decrease) in fuel-related costs														$ (42.9)
Change in requested rate increase (decrease) in non-fuel related revenues																(0.2)
Requested rate increase (decrease) in non-fuel related revenue									121.5							$ 121.5
Proposed increase (decrease) in non-fuel revenue									$ 41.3
Proposed return on equity									9.575%
Approved rate increase (decrease)								$ 61.2
Estimated period of time for New Mexico Supreme Court Appeal decision							15 months									7 months
Minimum amount of loss of capital cost recovery							15 months
Increase (decrease) of income tax expense																$ (2.1)
Estimate of possible loss				151.1												151.1
Public Service Company of New Mexico | 2015 Electric Rate Case | San Juan Generating Station | Refined Coal
Public Utilities, General Disclosures [Line Items]
Proposed credit to retail customers									100.00%
Public Service Company of New Mexico | 2015 Electric Rate Case | Clean Air Act, Balanced Draft Technology | San Juan Generating Station Units 1 and 4
Public Utilities, General Disclosures [Line Items]
Amount of equipment costs included in rate base									$ 40.0
Public Service Company of New Mexico | 2015 Electric Rate Case | Palo Verde Nuclear Generating Station, Unit 2 Leases
Public Utilities, General Disclosures [Line Items]
Proposed disallowance of recovery of purchase price of assets									163.3
Approved lease acquisition costs								83.7
Disallowed leasehold improvements								43.8
Pre-tax regulatory disallowance for capital costs				$ 3.1			$ 6.8
Public Service Company of New Mexico | 2015 Electric Rate Case | Palo Verde Nuclear Generating Station, Unit 1 Leases, extended
Public Utilities, General Disclosures [Line Items]
Approved recovery of assumed operating and maintenance expense savings annually related to BDT								$ 0.3
Public Service Company of New Mexico | 2015 Electric Rate Case | Alvarado square
Public Utilities, General Disclosures [Line Items]
Proposed disallowance of recovery of assets and deferred charges from retail customers									$ 4.5
Pre-tax regulatory disallowance of costs recorded as regulatory assets and deferred charges																$ 4.5
Public Service Company of New Mexico | 2015 Electric Rate Case | Non-Fuel Energy
Public Utilities, General Disclosures [Line Items]
Requested rate increase (decrease)											$ 121.7
Four Corners
Public Utilities, General Disclosures [Line Items]
Pre-tax regulatory disallowance																		$ 90.1	$ 148.1
Scenario, forecast
Public Utilities, General Disclosures [Line Items]
Pre-tax regulatory disallowance for capital costs															$ 36.8		$ 58.0