Regulatory Matters - Schedule of Regulatory Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2022	Dec. 31, 2021
Detail of regulatory liabilities
Current	$ 420,357	$ 296,271
Non-Current	2,108,289	2,499,213
Other postretirement benefits
Detail of regulatory liabilities
Current	50,624	37,789
Non-Current	292,490	337,027
Asset retirement obligations
Detail of regulatory liabilities
Current	0	0
Non-Current	316,513	614,683
Deferred fuel and purchased power — mark-to-market (Note 7)
Detail of regulatory liabilities
Current	172,946	60,693
Non-Current	86,243	46,908
Removal costs
Detail of regulatory liabilities
Current	72,955	69,476
Non-Current	35,619	50,104
Income taxes — change in rates
Detail of regulatory liabilities
Current	2,876	2,876
Non-Current	61,642	64,802
Four Corners coal reclamation
Detail of regulatory liabilities
Current	2,316	2,316
Non-Current	50,013	53,076
Income taxes — deferred investment tax credit
Detail of regulatory liabilities
Current	2,264	2,264
Non-Current	45,436	47,337
Spent nuclear fuel
Detail of regulatory liabilities
Current	7,224	6,701
Non-Current	33,377	38,581
Renewable energy standard (b)
Detail of regulatory liabilities
Current	29,626	38,453
Non-Current	661	187
FERC transmission true up (b)
Detail of regulatory liabilities
Current	21,354	21,379
Non-Current	659	12,924
Property tax deferral (e)
Detail of regulatory liabilities
Current	4,671	4,671
Non-Current	12,018	15,521
Sundance maintenance
Detail of regulatory liabilities
Current	0	0
Non-Current	16,119	13,797
Demand side management (b)
Detail of regulatory liabilities
Current	4,759	0
Non-Current	9,214	5,417
Tax expense adjustor mechanism (b) (e)
Detail of regulatory liabilities
Current	0	0
Non-Current	4,835	4,835
Other
Detail of regulatory liabilities
Current	600	1,511
Non-Current	2,998	592
ACC | Excess deferred income taxes - Tax Act
Detail of regulatory liabilities
Current	40,903	40,903
Non-Current	926,539	971,545
FERC | Excess deferred income taxes - Tax Act
Detail of regulatory liabilities
Current	7,239	7,239
Non-Current	$ 213,913	$ 221,877