Regulatory Matters - Schedule of Regulatory Liabilities (Details) - USD ($) $ in Thousands	Jun. 30, 2023	Dec. 31, 2022
Detail of regulatory liabilities
Regulatory liability, including deferred fuel and purchased power regulatory liability current	$ 2,251,600	$ 2,333,351
Less: current regulatory liabilities	219,075	271,575
Total non-current regulatory liabilities	2,032,525	2,061,776
Asset retirement obligations
Detail of regulatory liabilities
Regulatory liability, including deferred fuel and purchased power regulatory liability current	407,028	354,002
Other postretirement benefits
Detail of regulatory liabilities
Regulatory liability, including deferred fuel and purchased power regulatory liability current	246,580	270,604
Removal costs
Detail of regulatory liabilities
Regulatory liability, including deferred fuel and purchased power regulatory liability current	102,245	106,889
Income taxes — change in rates
Detail of regulatory liabilities
Regulatory liability, including deferred fuel and purchased power regulatory liability current	64,098	64,806
Income taxes — deferred investment tax credit
Detail of regulatory liabilities
Regulatory liability, including deferred fuel and purchased power regulatory liability current	59,359	48,035
Four Corners coal reclamation
Detail of regulatory liabilities
Regulatory liability, including deferred fuel and purchased power regulatory liability current	53,390	52,592
Spent nuclear fuel
Detail of regulatory liabilities
Regulatory liability, including deferred fuel and purchased power regulatory liability current	36,061	39,217
Renewable energy standard (b)
Detail of regulatory liabilities
Regulatory liability, including deferred fuel and purchased power regulatory liability current	29,524	35,720
Demand side management (b)
Detail of regulatory liabilities
Regulatory liability, including deferred fuel and purchased power regulatory liability current	22,236	8,461
Sundance maintenance
Detail of regulatory liabilities
Regulatory liability, including deferred fuel and purchased power regulatory liability current	18,441	16,893
Property tax deferral (e)
Detail of regulatory liabilities
Regulatory liability, including deferred fuel and purchased power regulatory liability current	13,186	15,521
FERC transmission true up (b)
Detail of regulatory liabilities
Regulatory liability, including deferred fuel and purchased power regulatory liability current	5,870	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	0	96,367
Other
Detail of regulatory liabilities
Regulatory liability, including deferred fuel and purchased power regulatory liability current	7,295	3,092
ACC | Excess deferred income taxes - Tax Act
Detail of regulatory liabilities
Regulatory liability, including deferred fuel and purchased power regulatory liability current	961,359	971,545
FERC | Excess deferred income taxes - Tax Act
Detail of regulatory liabilities
Regulatory liability, including deferred fuel and purchased power regulatory liability current	$ 220,093	$ 221,877