Regulatory Matters - Schedule of Regulatory Assets (Details) - USD ($) $ in Thousands	Sep. 30, 2025	Dec. 31, 2024
Detail of regulatory assets
Total regulatory assets	$ 1,806,935	$ 1,810,458
Less: current regulatory assets	335,625	420,969
Total non-current regulatory assets	1,471,310	1,389,489
Pension
Detail of regulatory assets
Total regulatory assets	722,942	750,976
Deferred fuel and purchased power
Detail of regulatory assets
Total regulatory assets	205,053	287,597
Income taxes — allowance for funds used during construction equity
Detail of regulatory assets
Total regulatory assets	204,422	192,936
Palo Verde sale leaseback noncontrolling interests acquisition (Note 8)
Detail of regulatory assets
Total regulatory assets	151,506	0
Ocotillo deferral
Detail of regulatory assets
Total regulatory assets	103,642	114,775
Lease incentives
Detail of regulatory assets
Total regulatory assets	91,192	70,541
SCR deferral
Detail of regulatory assets
Total regulatory assets	78,670	83,123
Retired power plant costs
Detail of regulatory assets
Total regulatory assets	57,799	68,380
FERC Transmission true up
Detail of regulatory assets
Total regulatory assets	39,095	35,159
Income taxes — investment tax credit basis adjustment
Detail of regulatory assets
Total regulatory assets	33,327	34,834
Deferred compensation
Detail of regulatory assets
Total regulatory assets	32,789	33,108
Deferred fuel and purchased power — mark-to-market (Note 9)
Detail of regulatory assets
Total regulatory assets	26,651	42,275
Deferred property taxes
Detail of regulatory assets
Total regulatory assets	17,491	23,918
Mead-Phoenix transmission line — contributions in aid of construction ("CIAC")
Detail of regulatory assets
Total regulatory assets	8,135	8,384
Palo Verde VIEs (Note 8)
Detail of regulatory assets
Total regulatory assets	8,095	20,611
Loss on reacquired debt
Detail of regulatory assets
Total regulatory assets	5,910	6,682
Power supply adjustor - interest
Detail of regulatory assets
Total regulatory assets	4,836	11,525
Active union medical trust
Detail of regulatory assets
Total regulatory assets	4,352	9,673
Tax expense adjustor mechanism
Detail of regulatory assets
Total regulatory assets	4,043	4,534
Navajo Coal reclamation
Detail of regulatory assets
Total regulatory assets	3,593	7,905
Other
Detail of regulatory assets
Total regulatory assets	$ 3,392	$ 3,522