Regulatory Matters - Schedule of Regulatory Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Detail of regulatory liabilities
Regulatory liabilities, current	$ 296,271	$ 229,088
Regulatory liabilities, non-current	2,499,213	2,450,169
Asset retirement obligations
Detail of regulatory liabilities
Regulatory liabilities, current	0	0
Regulatory liabilities, non-current	614,683	506,049
Other postretirement benefits
Detail of regulatory liabilities
Regulatory liabilities, current	37,789	37,705
Regulatory liabilities, non-current	337,027	349,588
Removal costs
Detail of regulatory liabilities
Regulatory liabilities, current	69,476	52,844
Regulatory liabilities, non-current	50,104	103,008
Deferred fuel and purchased power - mark-to-market
Detail of regulatory liabilities
Regulatory liabilities, current	60,693	0
Regulatory liabilities, non-current	46,908	0
Income taxes — change in rates
Detail of regulatory liabilities
Regulatory liabilities, current	2,876	2,839
Regulatory liabilities, non-current	64,802	66,553
Four Corners coal reclamation
Detail of regulatory liabilities
Regulatory liabilities, current	2,316	5,460
Regulatory liabilities, non-current	53,076	49,435
Spent nuclear fuel
Detail of regulatory liabilities
Regulatory liabilities, current	6,701	6,768
Regulatory liabilities, non-current	38,581	44,221
Income taxes — deferred investment tax credit
Detail of regulatory liabilities
Regulatory liabilities, current	2,264	2,231
Regulatory liabilities, non-current	47,337	48,648
Renewable energy program
Detail of regulatory liabilities
Regulatory liabilities, current	38,453	39,442
Regulatory liabilities, non-current	187	103
FERC transmission true up
Detail of regulatory liabilities
Regulatory liabilities, current	21,379	6,598
Regulatory liabilities, non-current	12,924	3,008
Property tax deferral
Detail of regulatory liabilities
Regulatory liabilities, current	4,671	0
Regulatory liabilities, non-current	15,521	13,856
Sundance maintenance
Detail of regulatory liabilities
Regulatory liabilities, current	0	2,989
Regulatory liabilities, non-current	13,797	11,508
Demand side management
Detail of regulatory liabilities
Regulatory liabilities, current	0	10,819
Regulatory liabilities, non-current	5,417	0
Tax expense adjustor mechanism
Detail of regulatory liabilities
Regulatory liabilities, current	0	7,089
Regulatory liabilities, non-current	4,835	0
Deferred gains on utility property
Detail of regulatory liabilities
Regulatory liabilities, current	1,301	2,423
Regulatory liabilities, non-current	551	1,544
TCA balancing account
Detail of regulatory liabilities
Regulatory liabilities, current	0	2,902
Regulatory liabilities, non-current	0	4,672
Active union medical trust
Detail of regulatory liabilities
Regulatory liabilities, current	0	0
Regulatory liabilities, non-current	0	6,057
Other
Detail of regulatory liabilities
Regulatory liabilities, current	210	409
Regulatory liabilities, non-current	41	189
ACC | Excess deferred income taxes - ACC - Tax Cuts and Jobs Act
Detail of regulatory liabilities
Regulatory liabilities, current	40,903	41,330
Regulatory liabilities, non-current	971,545	1,012,583
FERC | Excess deferred income taxes - ACC - Tax Cuts and Jobs Act
Detail of regulatory liabilities
Regulatory liabilities, current	7,239	7,240
Regulatory liabilities, non-current	$ 221,877	$ 229,147