Equity Method Investment - Summarized Financial Information (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Income Statement [Abstract]
Revenues	$ 472,465	$ 433,586	$ 1,163,736	$ 1,113,458
Balance Sheet Related Disclosures [Abstract]
Construction work in progress	379,848		379,848		$ 161,106
Assets	7,742,112	7,200,932	7,742,112	7,200,932	7,298,774
Total stockholders' equity	1,773,316		1,773,316		1,678,698
NMRD
Income Statement [Abstract]
Revenues	3,462	892	7,769	2,720
Operating Expenses	2,100	730	5,152	2,181
Net Earnings Attributable to PNMR	1,362	$ 162	2,617	$ 539
Balance Sheet Related Disclosures [Abstract]
Current assets	6,527		6,527		7,187
Construction work in progress	173,952		173,952		132,772
Non-current assets	1,969		1,969		0
Assets	182,448		182,448		139,959
Current liabilities	1,059		1,059		9,640
Non-current liabilities	351		351		0
Total stockholders' equity	$ 181,038		$ 181,038		$ 130,319