Regulatory Matters - Schedule of Regulatory Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
Detail of regulatory liabilities
Regulatory liability, including deferred fuel and purchased power regulatory liability current	$ 2,146,270	$ 2,333,351
Regulatory liabilities (Note 4)	226,989	271,575
Total non-current regulatory liabilities	1,919,281	2,061,776
Asset retirement obligations
Detail of regulatory liabilities
Regulatory liability, including deferred fuel and purchased power regulatory liability current	343,998	354,002
Other postretirement benefits
Detail of regulatory liabilities
Regulatory liability, including deferred fuel and purchased power regulatory liability current	234,832	270,604
Removal costs
Detail of regulatory liabilities
Regulatory liability, including deferred fuel and purchased power regulatory liability current	96,698	106,889
Income taxes — change in rates
Detail of regulatory liabilities
Regulatory liability, including deferred fuel and purchased power regulatory liability current	61,879	64,806
Income taxes — deferred investment tax credit
Detail of regulatory liabilities
Regulatory liability, including deferred fuel and purchased power regulatory liability current	67,440	48,035
Four Corners coal reclamation
Detail of regulatory liabilities
Regulatory liability, including deferred fuel and purchased power regulatory liability current	54,275	52,592
Spent nuclear fuel
Detail of regulatory liabilities
Regulatory liability, including deferred fuel and purchased power regulatory liability current	33,616	39,217
Renewable energy standard (b)
Detail of regulatory liabilities
Regulatory liability, including deferred fuel and purchased power regulatory liability current	37,624	35,720
Demand side management (b)
Detail of regulatory liabilities
Regulatory liability, including deferred fuel and purchased power regulatory liability current	23,706	8,461
Sundance maintenance
Detail of regulatory liabilities
Regulatory liability, including deferred fuel and purchased power regulatory liability current	19,215	16,893
Property tax deferral (e)
Detail of regulatory liabilities
Regulatory liability, including deferred fuel and purchased power regulatory liability current	12,018	15,521
FERC transmission true up (b)
Detail of regulatory liabilities
Regulatory liability, including deferred fuel and purchased power regulatory liability current	6,087	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	6,128	3,092
ACC | Excess deferred income taxes - Tax Act
Detail of regulatory liabilities
Regulatory liability, including deferred fuel and purchased power regulatory liability current	929,420	971,545
FERC | Excess deferred income taxes - Tax Act
Detail of regulatory liabilities
Regulatory liability, including deferred fuel and purchased power regulatory liability current	$ 214,499	$ 221,877