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Stock-Based Compensation Plans Weighted Average Assumptions (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Restricted Stock and Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected quarterly dividends per share, in dollars per share	$ 0.185pnm_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedQuarterlyDividendRate / us-gaap_AwardTypeAxis = pnm_RestrictedStockAndPerformanceSharesMember	$ 0.165pnm_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedQuarterlyDividendRate / us-gaap_AwardTypeAxis = pnm_RestrictedStockAndPerformanceSharesMember	$ 0.145pnm_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedQuarterlyDividendRate / us-gaap_AwardTypeAxis = pnm_RestrictedStockAndPerformanceSharesMember
Risk-free interest rates, percentage	0.62%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate / us-gaap_AwardTypeAxis = pnm_RestrictedStockAndPerformanceSharesMember	0.34%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate / us-gaap_AwardTypeAxis = pnm_RestrictedStockAndPerformanceSharesMember	1.22%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate / us-gaap_AwardTypeAxis = pnm_RestrictedStockAndPerformanceSharesMember
Market-Based Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rates, percentage	0.64%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate / us-gaap_AwardTypeAxis = pnm_MarketBasedSharesMember	0.36%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate / us-gaap_AwardTypeAxis = pnm_MarketBasedSharesMember	1.04%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate / us-gaap_AwardTypeAxis = pnm_MarketBasedSharesMember
Dividend yield	2.82%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate / us-gaap_AwardTypeAxis = pnm_MarketBasedSharesMember	2.86%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate / us-gaap_AwardTypeAxis = pnm_MarketBasedSharesMember	3.45%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate / us-gaap_AwardTypeAxis = pnm_MarketBasedSharesMember
Expected volatility	25.11%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate / us-gaap_AwardTypeAxis = pnm_MarketBasedSharesMember	25.11%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate / us-gaap_AwardTypeAxis = pnm_MarketBasedSharesMember	43.98%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate / us-gaap_AwardTypeAxis = pnm_MarketBasedSharesMember

X

- Definition

Share Based Compensation Arrangement by Share Based Payment Award Fair Value Assumptions Expected Quarterly Dividend Rate

+ References

No definition available.

+ Details

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- Definition

The estimated dividend rate (a percentage of the share price) to be paid (expected dividends) to holders of the underlying shares over the option's term.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (f)(2)(iii)

-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

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- Definition

The estimated measure of the percentage by which a share price is expected to fluctuate during a period. Volatility also may be defined as a probability-weighted measure of the dispersion of returns about the mean. The volatility of a share price is the standard deviation of the continuously compounded rates of return on the share over a specified period. That is the same as the standard deviation of the differences in the natural logarithms of the stock prices plus dividends, if any, over the period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (f)(2)(ii)

-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

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- Definition

The risk-free interest rate assumption that is used in valuing an option on its own shares.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (f)(2)(iv)

-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

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