Schedule I - Condensed Financial Information of Parent Company - Balance Sheets (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Assets
Cash and cash equivalents	$ 3,974	$ 4,522	$ 46,051	$ 28,274
Income taxes receivable	6,879	6,066
Other, net	47,358	73,444
Total current assets	294,420	378,039
Property, plant and equipment, net of accumulated depreciation of $13,229 and $12,291	4,980,227	4,904,715
Other long-term assets	503	547
Total assets	6,646,103	6,471,080	6,009,328
Liabilities and Stockholders’ Equity
Short-term debt	305,400	287,100
Current maturities of long-term debt	256,895	273,348
Accrued interest and taxes	62,357	61,871
Other current liabilities	53,850	59,314
Total current liabilities	835,644	805,108
Long-term debt	2,180,750	2,119,364
Total liabilities	4,873,126	4,714,679
Common stock (no par value; 120,000,000 shares authorized; issued and outstanding 79,653,624 shares)	1,157,665	1,163,661
Accumulated other comprehensive income (loss), net of tax	(95,940)	(92,451)
Retained earnings	633,528	604,742
Total PNMR common stockholders’ equity	1,695,253	1,675,952
Total liabilities and stockholders' equity	6,646,103	6,471,080
PNM Resources
Assets
Cash and cash equivalents	21	54	$ 441	$ 382
Intercompany receivables	96,227	92,234
Income taxes receivable	1,818	0
Other, net	1,937	233
Total current assets	100,003	92,521
Property, plant and equipment, net of accumulated depreciation of $13,229 and $12,291	26,546	26,366
Investment in subsidiaries	2,056,198	1,986,276
Other long-term assets	66,090	79,314
Total long-term assets	2,148,834	2,091,956
Total assets	2,248,837	2,184,477
Liabilities and Stockholders’ Equity
Short-term debt	265,600	226,100
Short-term debt-affiliate	11,919	8,819
Current maturities of long-term debt	249,979	0
Accrued interest and taxes	1,661	1,333
Other current liabilities	21,274	19,374
Total current liabilities	550,433	255,626
Long-term debt	0	249,895
Other long-term liabilities	3,151	3,004
Total liabilities	553,584	508,525
Common stock (no par value; 120,000,000 shares authorized; issued and outstanding 79,653,624 shares)	1,157,665	1,163,661
Accumulated other comprehensive income (loss), net of tax	(95,940)	(92,451)
Retained earnings	633,528	604,742
Total PNMR common stockholders’ equity	1,695,253	1,675,952
Total liabilities and stockholders' equity	$ 2,248,837	$ 2,184,477