Regulatory and Rate Matters - New Mexico General Rate Cases (Details) $ in Millions									9 Months Ended	12 Months Ended
	Dec. 07, 2016 USD ($)	Sep. 30, 2016	Sep. 28, 2016 USD ($) 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 ($)	Dec. 31, 2016 USD ($)	May 31, 2016 MW
NMPRC [Member]
Public Utilities, General Disclosures [Line Items]
Requested rate increase (decrease)	$ 99.2
Requested return on equity	10.125%
Public Service Company of New Mexico [Member] | Clean Air Act, Balanced Draft Technology [Member] | San Juan Generating Station Units 1 and 4 [Member]
Public Utilities, General Disclosures [Line Items]
Net book value										$ 50.0
Public Service Company of New Mexico [Member] | Clean Air Act, Balanced Draft Technology [Member] | San Juan Generating Station Units 1 and 4 [Member] | Installation Costs Including Construction Management, Gross Receipts Taxes, AFUDC, and Other PNM Costs [Member]
Public Utilities, General Disclosures [Line Items]
Portion of costs for SNCRs and BDT equipment				$ 52.3
Public Service Company of New Mexico [Member] | Palo Verde Nuclear Generating Station, Unit 2 Leases [Member]
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										76.9
Net book value of capital improvements										39.9
Public Service Company of New Mexico [Member] | Palo Verde Nuclear Generating Station, Unit 1 Leases, extended [Member]
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 [Member] | 2014 Electric Rate Case [Member]
Public Utilities, General Disclosures [Line Items]
Requested rate increase (decrease)								$ 107.4
Public Service Company of New Mexico [Member] | 2015 Electric Rate Case [Member]
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
Minimum amount of loss of capital cost recovery		15 months
Increase (decrease) of income tax expense										(2.1)
Estimate of possible loss										155.6
Public Service Company of New Mexico [Member] | 2015 Electric Rate Case [Member] | San Juan Generating Station [Member] | Refined Coal [Member]
Public Utilities, General Disclosures [Line Items]
Proposed credit to retail customers				100.00%
Public Service Company of New Mexico [Member] | 2015 Electric Rate Case [Member] | Clean Air Act, Balanced Draft Technology [Member] | San Juan Generating Station Units 1 and 4 [Member]
Public Utilities, General Disclosures [Line Items]
Amount of equipment costs included in rate base				$ 40.0
Public Service Company of New Mexico [Member] | 2015 Electric Rate Case [Member] | Palo Verde Nuclear Generating Station, Unit 2 Leases [Member]
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										6.8
Public Service Company of New Mexico [Member] | 2015 Electric Rate Case [Member] | Palo Verde Nuclear Generating Station, Unit 1 Leases, extended [Member]
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 [Member] | 2015 Electric Rate Case [Member] | Alvarado square [Member]
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 [Member] | 2015 Electric Rate Case [Member] | Non-Fuel Energy [Member]
Public Utilities, General Disclosures [Line Items]
Requested rate increase (decrease)						$ 121.7