Regulatory Matters - Schedule of Regulatory Liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2023	Dec. 31, 2022
Detail of regulatory liabilities
Regulatory liability, including deferred fuel and purchased power regulatory liability current	$ 2,262,539	$ 2,333,351
Less: current regulatory liabilities	202,266	271,575
Total non-current regulatory liabilities	2,060,273	2,061,776
Asset retirement obligations
Detail of regulatory liabilities
Regulatory liability, including deferred fuel and purchased power regulatory liability current	396,107	354,002
Other postretirement benefits
Detail of regulatory liabilities
Regulatory liability, including deferred fuel and purchased power regulatory liability current	258,955	270,604
Removal costs
Detail of regulatory liabilities
Regulatory liability, including deferred fuel and purchased power regulatory liability current	106,102	106,889
Income taxes — change in rates
Detail of regulatory liabilities
Regulatory liability, including deferred fuel and purchased power regulatory liability current	64,739	64,806
Four Corners coal reclamation
Detail of regulatory liabilities
Regulatory liability, including deferred fuel and purchased power regulatory liability current	53,348	52,592
Income taxes — deferred investment tax credit
Detail of regulatory liabilities
Regulatory liability, including deferred fuel and purchased power regulatory liability current	49,031	48,035
Spent nuclear fuel
Detail of regulatory liabilities
Regulatory liability, including deferred fuel and purchased power regulatory liability current	37,313	39,217
Renewable energy standard (b)
Detail of regulatory liabilities
Regulatory liability, including deferred fuel and purchased power regulatory liability current	30,188	35,720
Sundance maintenance
Detail of regulatory liabilities
Regulatory liability, including deferred fuel and purchased power regulatory liability current	17,667	16,893
Property tax deferral (e)
Detail of regulatory liabilities
Regulatory liability, including deferred fuel and purchased power regulatory liability current	14,353	15,521
Demand side management (b)
Detail of regulatory liabilities
Regulatory liability, including deferred fuel and purchased power regulatory liability current	14,207	8,461
FERC transmission true up (b)
Detail of regulatory liabilities
Regulatory liability, including deferred fuel and purchased power regulatory liability current	12,597	22,895
Tax expense adjustor mechanism (b) (e)
Detail of regulatory liabilities
Regulatory liability, including deferred fuel and purchased power regulatory liability current	4,835	4,835
Deferred fuel and purchased power — mark-to-market (Note 7)
Detail of regulatory liabilities
Regulatory liability, including deferred fuel and purchased power regulatory liability current	3,396	96,367
Other
Detail of regulatory liabilities
Regulatory liability, including deferred fuel and purchased power regulatory liability current	7,415	3,092
ACC | Excess deferred income taxes - Tax Act
Detail of regulatory liabilities
Regulatory liability, including deferred fuel and purchased power regulatory liability current	970,578	971,545
FERC | Excess deferred income taxes - Tax Act
Detail of regulatory liabilities
Regulatory liability, including deferred fuel and purchased power regulatory liability current	$ 221,708	$ 221,877