Regulatory Matters - Schedule of Regulatory Assets (Details) - USD ($) $ in Thousands	Sep. 30, 2024	Dec. 31, 2023
Detail of regulatory assets
Total regulatory assets	$ 1,863,144	$ 2,016,036
Less: current regulatory assets	517,367	625,757
Total non-current regulatory assets	1,345,777	1,390,279
Pension
Detail of regulatory assets
Total regulatory assets	678,036	696,476
Deferred fuel and purchased power
Detail of regulatory assets
Total regulatory assets	367,274	463,195
Income taxes — allowance for funds used during construction (“AFUDC”) equity
Detail of regulatory assets
Total regulatory assets	193,283	189,058
Ocotillo deferral
Detail of regulatory assets
Total regulatory assets	118,486	128,636
Deferred fuel and purchased power — mark-to-market (Note 7)
Detail of regulatory assets
Total regulatory assets	99,026	120,214
SCR deferral
Detail of regulatory assets
Total regulatory assets	84,607	89,477
Retired power plant costs
Detail of regulatory assets
Total regulatory assets	72,169	83,536
Lease incentives
Detail of regulatory assets
Total regulatory assets	63,337	46,615
Income taxes — investment tax credit basis adjustment
Detail of regulatory assets
Total regulatory assets	34,573	34,230
Deferred compensation
Detail of regulatory assets
Total regulatory assets	33,923	33,972
Deferred property taxes
Detail of regulatory assets
Total regulatory assets	26,061	32,488
FERC Transmission true up
Detail of regulatory assets
Total regulatory assets	21,039	616
Palo Verde VIEs (Note 6)
Detail of regulatory assets
Total regulatory assets	20,651	20,772
Power supply adjustor - interest
Detail of regulatory assets
Total regulatory assets	8,952	19,416
Navajo coal reclamation
Detail of regulatory assets
Total regulatory assets	8,649	10,883
Mead-Phoenix transmission line contributions in aid of construction (“CIAC”)
Detail of regulatory assets
Total regulatory assets	8,467	8,716
Active Union Medical Trust
Detail of regulatory assets
Total regulatory assets	7,480	12,747
Loss on reacquired debt
Detail of regulatory assets
Total regulatory assets	7,003	7,965
Tax expense adjustor mechanism
Detail of regulatory assets
Total regulatory assets	4,698	5,190
Four Corners cost deferral
Detail of regulatory assets
Total regulatory assets	1,865	7,922
Other
Detail of regulatory assets
Total regulatory assets	$ 3,565	$ 3,912