Regulatory Matters - Schedule of Regulatory Liabilities (Details) - USD ($) $ in Thousands	Jun. 30, 2022	Dec. 31, 2021
Detail of regulatory liabilities
Current	$ 404,107	$ 296,271
Non-Current	2,268,148	2,499,213
Asset retirement obligations
Detail of regulatory liabilities
Current	0	0
Non-Current	376,575	614,683
Other postretirement benefits
Detail of regulatory liabilities
Current	50,624	37,789
Non-Current	305,045	337,027
Deferred fuel and purchased power — mark-to-market (Note 7)
Detail of regulatory liabilities
Current	173,712	60,693
Non-Current	111,847	46,908
Removal costs
Detail of regulatory liabilities
Current	57,454	69,476
Non-Current	55,570	50,104
Income taxes — change in rates
Detail of regulatory liabilities
Current	2,876	2,876
Non-Current	63,636	64,802
Four Corners coal reclamation
Detail of regulatory liabilities
Current	2,316	2,316
Non-Current	50,860	53,076
Income taxes — deferred investment tax credit
Detail of regulatory liabilities
Current	2,264	2,264
Non-Current	46,663	47,337
Spent nuclear fuel
Detail of regulatory liabilities
Current	6,730	6,701
Non-Current	35,227	38,581
FERC transmission true up (b)
Detail of regulatory liabilities
Current	29,238	21,379
Non-Current	3,620	12,924
Renewable energy standard
Detail of regulatory liabilities
Current	25,401	38,453
Non-Current	492	187
Property tax deferral (e)
Detail of regulatory liabilities
Current	4,671	4,671
Non-Current	13,186	15,521
Sundance maintenance
Detail of regulatory liabilities
Current	0	0
Non-Current	15,345	13,797
Demand side management
Detail of regulatory liabilities
Current	0	0
Non-Current	9,214	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	679	1,511
Non-Current	2,155	592
ACC | Excess deferred income taxes - Tax Act
Detail of regulatory liabilities
Current	40,903	40,903
Non-Current	954,939	971,545
FERC | Excess deferred income taxes - Tax Act
Detail of regulatory liabilities
Current	7,239	7,239
Non-Current	$ 218,939	$ 221,877