Regulatory Matters - Schedule of Regulatory Liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2022	Dec. 31, 2021
Detail of regulatory liabilities
Current	$ 448,778	$ 296,271
Non-Current	2,438,672	2,499,213
Asset retirement obligations
Detail of regulatory liabilities
Current	0	0
Non-Current	537,720	614,683
Other postretirement benefits
Detail of regulatory liabilities
Current	37,789	37,789
Non-Current	324,697	337,027
Deferred fuel and purchased power — mark-to-market (Note 7)
Detail of regulatory liabilities
Current	214,571	60,693
Non-Current	91,521	46,908
Removal costs
Detail of regulatory liabilities
Current	69,054	69,476
Non-Current	48,302	50,104
Income taxes — change in rates
Detail of regulatory liabilities
Current	2,876	2,876
Non-Current	64,655	64,802
Four Corners coal reclamation
Detail of regulatory liabilities
Current	2,316	2,316
Non-Current	51,629	53,076
Income taxes — deferred investment tax credit
Detail of regulatory liabilities
Current	2,264	2,264
Non-Current	47,253	47,337
Spent nuclear fuel
Detail of regulatory liabilities
Current	6,631	6,701
Non-Current	37,136	38,581
Renewable energy standard
Detail of regulatory liabilities
Current	30,729	38,453
Non-Current	452	187
FERC transmission true up (b)
Detail of regulatory liabilities
Current	27,595	21,379
Non-Current	375	12,924
Property tax deferral (e)
Detail of regulatory liabilities
Current	4,671	4,671
Non-Current	14,353	15,521
Sundance maintenance
Detail of regulatory liabilities
Current	0	0
Non-Current	14,571	13,797
Demand side management
Detail of regulatory liabilities
Current	1,111	0
Non-Current	9,216	5,417
Tax expense adjustor mechanism
Detail of regulatory liabilities
Current	0	0
Non-Current	4,835	4,835
Other
Detail of regulatory liabilities
Current	1,029	1,511
Non-Current	990	592
ACC | Excess deferred income taxes - Tax Act
Detail of regulatory liabilities
Current	40,903	40,903
Non-Current	969,459	971,545
FERC | Excess deferred income taxes - Tax Act
Detail of regulatory liabilities
Current	7,239	7,239
Non-Current	$ 221,508	$ 221,877