Regulatory Matters - Schedule of Regulatory Assets (Details) - USD ($) $ in Thousands	Mar. 31, 2026	Dec. 31, 2025
Detail of regulatory assets
Total regulatory assets	$ 1,650,939	$ 1,749,366
Less: current regulatory assets	196,729	286,009
Total non-current regulatory assets	1,454,210	1,463,357
Pension
Detail of regulatory assets
Total regulatory assets	712,420	723,042
Income taxes — AFUDC equity
Detail of regulatory assets
Total regulatory assets	205,059	203,890
Palo Verde sale leaseback noncontrolling interests acquisition
Detail of regulatory assets
Total regulatory assets	151,506	151,506
Ocotillo deferral
Detail of regulatory assets
Total regulatory assets	96,220	99,931
Lease incentive (Note 17)
Detail of regulatory assets
Total regulatory assets	88,817	90,005
SCR deferral
Detail of regulatory assets
Total regulatory assets	75,816	77,186
Deferred fuel and purchased power — mark-to-market (Note 10)
Detail of regulatory assets
Total regulatory assets	58,222	29,330
Deferred fuel and purchased power
Detail of regulatory assets
Total regulatory assets	55,432	149,068
Retired power plant costs
Detail of regulatory assets
Total regulatory assets	50,695	56,809
Income taxes — investment tax credit basis adjustment (Note 5)
Detail of regulatory assets
Total regulatory assets	42,300	42,459
Deferred compensation
Detail of regulatory assets
Total regulatory assets	32,936	32,204
FERC transmission true up
Detail of regulatory assets
Total regulatory assets	31,138	21,471
Deferred property taxes
Detail of regulatory assets
Total regulatory assets	13,207	15,349
Palo Verde VIEs (Note 9)
Detail of regulatory assets
Total regulatory assets	8,834	8,582
Mead-Phoenix transmission line — contributions in aid of construction
Detail of regulatory assets
Total regulatory assets	7,969	8,052
Loss on reacquired debt
Detail of regulatory assets
Total regulatory assets	5,416	5,653
Active union medical trust
Detail of regulatory assets
Total regulatory assets	4,375	3,696
TEAM
Detail of regulatory assets
Total regulatory assets	3,715	3,879
Navajo coal reclamation
Detail of regulatory assets
Total regulatory assets	1,438	2,516
PSA - interest
Detail of regulatory assets
Total regulatory assets	681	5,679
DSM
Detail of regulatory assets
Total regulatory assets	206	15,706
Other
Detail of regulatory assets
Total regulatory assets	$ 4,537	$ 3,353