Regulatory Matters - Schedule of Regulatory Assets (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023
Detail of regulatory assets
Total regulatory assets	$ 1,950,129	$ 2,016,036
Less: current regulatory assets	578,869	625,757
Total non-current regulatory assets	1,371,260	1,390,279
Pension
Detail of regulatory assets
Total regulatory assets	686,379	696,476
Deferred fuel and purchased power
Detail of regulatory assets
Total regulatory assets	387,737	463,195
Income taxes — allowance for funds used during construction (“AFUDC”) equity
Detail of regulatory assets
Total regulatory assets	189,211	189,058
Deferred fuel and purchased power — mark-to-market (Note 7)
Detail of regulatory assets
Total regulatory assets	154,658	120,214
Ocotillo deferral
Detail of regulatory assets
Total regulatory assets	125,908	128,636
SCR deferral
Detail of regulatory assets
Total regulatory assets	87,576	89,477
Retired power plant costs
Detail of regulatory assets
Total regulatory assets	79,747	83,536
Lease incentives
Detail of regulatory assets
Total regulatory assets	54,543	46,615
Income taxes — investment tax credit basis adjustment
Detail of regulatory assets
Total regulatory assets	36,017	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	30,345	32,488
Palo Verde VIEs (Note 6)
Detail of regulatory assets
Total regulatory assets	20,732	20,772
Active Union Medical Trust
Detail of regulatory assets
Total regulatory assets	13,809	12,747
Navajo coal reclamation
Detail of regulatory assets
Total regulatory assets	10,139	10,883
Mead-Phoenix transmission line contributions in aid of construction (“CIAC”)
Detail of regulatory assets
Total regulatory assets	8,633	8,716
Loss on reacquired debt
Detail of regulatory assets
Total regulatory assets	7,644	7,965
Four Corners cost deferral
Detail of regulatory assets
Total regulatory assets	5,903	7,922
Tax expense adjustor mechanism
Detail of regulatory assets
Total regulatory assets	5,026	5,190
FERC Transmission true up
Detail of regulatory assets
Total regulatory assets	4,313	616
Power supply adjustor - interest
Detail of regulatory assets
Total regulatory assets	3,105	19,416
Other
Detail of regulatory assets
Total regulatory assets	$ 4,781	$ 3,912