Regulatory Matters - Schedule of Regulatory Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2025	Dec. 31, 2024
Detail of regulatory liabilities
Total regulatory liabilities	$ 1,963,994	$ 2,062,233
Less: current regulatory liabilities	194,094	206,955
Total non-current regulatory liabilities	1,769,900	1,855,278
Asset retirement obligations and removal costs
Detail of regulatory liabilities
Total regulatory liabilities	314,510	358,403
Other postretirement benefits
Detail of regulatory liabilities
Total regulatory liabilities	226,043	238,113
Four Corners coal reclamation
Detail of regulatory liabilities
Total regulatory liabilities	97,857	77,532
Income taxes — deferred investment tax credit
Detail of regulatory liabilities
Total regulatory liabilities	63,663	66,327
Income taxes — change in rates
Detail of regulatory liabilities
Total regulatory liabilities	56,421	59,133
Renewable energy standard
Detail of regulatory liabilities
Total regulatory liabilities	43,190	68,523
Demand side management
Detail of regulatory liabilities
Total regulatory liabilities	30,591	23,927
Sundance maintenance
Detail of regulatory liabilities
Total regulatory liabilities	25,408	23,086
Spent nuclear fuel
Detail of regulatory liabilities
Total regulatory liabilities	22,357	26,818
Deferred fuel and purchased power — mark-to-market (Note 9)
Detail of regulatory liabilities
Total regulatory liabilities	15,600	0
TCA Balancing Account
Detail of regulatory liabilities
Total regulatory liabilities	10,413	14,834
Tax expense adjustor mechanism
Detail of regulatory liabilities
Total regulatory liabilities	3,889	4,343
Property tax deferral
Detail of regulatory liabilities
Total regulatory liabilities	3,143	4,785
Other
Detail of regulatory liabilities
Total regulatory liabilities	456	113
ACC | Excess deferred income taxes - Tax Cuts and Jobs Act
Detail of regulatory liabilities
Total regulatory liabilities	849,886	888,896
FERC | Excess deferred income taxes - Tax Cuts and Jobs Act
Detail of regulatory liabilities
Total regulatory liabilities	$ 200,567	$ 207,400