Pension and Other Postretirement Benefit Plans Narrative (Details) (USD $)	12 Months Ended					9 Months Ended	12 Months Ended				9 Months Ended	12 Months Ended										1 Months Ended
	Dec. 31, 2012	Dec. 31, 2012 Pension Plan [Member]	Dec. 31, 2012 Other Postretirement Benefits [Member]	Dec. 31, 2012 Public Service Company of New Mexico [Member] Pension Plan [Member]	Dec. 31, 2011 Public Service Company of New Mexico [Member] Pension Plan [Member]	Sep. 30, 2012 Public Service Company of New Mexico [Member] Other Postretirement Benefits [Member]	Dec. 31, 2012 Public Service Company of New Mexico [Member] Other Postretirement Benefits [Member]	Dec. 31, 2011 Public Service Company of New Mexico [Member] Other Postretirement Benefits [Member]	Dec. 31, 2012 Texas-New Mexico Power Company [Member] Pension Plan [Member]	Dec. 31, 2011 Texas-New Mexico Power Company [Member] Pension Plan [Member]	Sep. 30, 2012 Texas-New Mexico Power Company [Member] Other Postretirement Benefits [Member]	Dec. 31, 2012 Texas-New Mexico Power Company [Member] Other Postretirement Benefits [Member]	Dec. 31, 2011 Texas-New Mexico Power Company [Member] Other Postretirement Benefits [Member]	Dec. 31, 2012 Equity Securities [Member] Pension Plan [Member]	Dec. 31, 2012 Equity Securities [Member] Other Postretirement Benefits [Member]	Dec. 31, 2012 Debt Securities [Member] Pension Plan [Member]	Dec. 31, 2012 Debt Securities [Member] Other Postretirement Benefits [Member]	Dec. 31, 2012 Alternative Investments [Member] Pension Plan [Member]	Dec. 31, 2012 Developed Countries Outside of United States [Member] Equity Securities [Member] Pension Plan [Member]	Dec. 31, 2012 Minimum [Member] Pension Plan [Member]	Dec. 31, 2012 Maximum [Member] Pension Plan [Member]	Jan. 31, 2013 Subsequent Event [Member] Public Service Company of New Mexico [Member] Pension Plan [Member]	Jan. 31, 2013 Subsequent Event [Member] Texas-New Mexico Power Company [Member] Pension Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plans, Gains and losses that lie outside the corridor, amortized (in years)	5 years
Benefit obligation, period increase (decrease)				$ 86,400,000	$ 2,800,000	$ 13,100,000		$ (1,000,000)	$ 10,700,000	$ (1,200,000)	$ 2,000,000		$ (100,000)
Defined Benefit Plan, Benefit Obligation, Period Increase (Decrease) Due to Change in Demographics				(8,000,000)	1,600,000	8,100,000		(1,700,000)	(800,000)	1,700,000	(800,000)		600,000
Actual plan asset allocations (as a percent)				11.00%			16.30%		11.20%			16.10%
Expected long-term return resulting from effect of one-percentage point decrease (as a percent)		1.00%	1.00%
Expected Long-term return on assets decrease resulting in increase net periodic costs In next fiscal year				5,500,000			600,000		600,000			100,000
Expected long-term return resulting from effect of one-percentage point increase (as a percent)		1.00%
Target plan asset allocations (as a percent)														31.00%	70.00%	53.00%	30.00%	16.00%	8.00%
Employer contributions				77,700,000	41,511,000		3,529,000	2,873,000	5,300,000	1,151,000		342,000	351,000									60,000,000	1,000,000
Estimated future employer contributions from 2013-2017				49,100,000					0
Weighted average discount rate related to anticipated contributions (as a percent)																				4.80%	5.20%
Early Retirement Portion Relating to Regulated Operations										100.00%
Estimated future employer contributions in next fiscal year							$ 3,500,000					$ 300,000