Regulatory Matters - Schedule of Regulatory Assets (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
Detail of regulatory assets
Regulatory asset including deferred fuel and purchased power regulatory asset current	$ 1,933,563	$ 1,822,100
Less: current regulatory assets	630,319	538,879
Total non-current regulatory assets	1,303,244	1,283,221
Pension
Detail of regulatory assets
Regulatory asset including deferred fuel and purchased power regulatory asset current	631,307	637,656
Deferred fuel and purchased power
Detail of regulatory assets
Regulatory asset including deferred fuel and purchased power regulatory asset current	526,666	460,561
Income taxes — allowance for funds used during construction (“AFUDC”) equity
Detail of regulatory assets
Regulatory asset including deferred fuel and purchased power regulatory asset current	188,845	179,631
Ocotillo deferral
Detail of regulatory assets
Regulatory asset including deferred fuel and purchased power regulatory asset current	131,013	138,143
SCR deferral
Detail of regulatory assets
Regulatory asset including deferred fuel and purchased power regulatory asset current	91,514	97,624
Retired power plant costs
Detail of regulatory assets
Regulatory asset including deferred fuel and purchased power regulatory asset current	87,325	98,692
Lease incentives
Detail of regulatory assets
Regulatory asset including deferred fuel and purchased power regulatory asset current	39,700	0
Deferred property taxes
Detail of regulatory assets
Regulatory asset including deferred fuel and purchased power regulatory asset current	34,630	41,057
Deferred compensation
Detail of regulatory assets
Regulatory asset including deferred fuel and purchased power regulatory asset current	34,492	33,660
Deferred fuel and purchased power — mark-to-market (Note 7)
Detail of regulatory assets
Regulatory asset including deferred fuel and purchased power regulatory asset current	34,131	0
Income taxes — investment tax credit basis adjustment
Detail of regulatory assets
Regulatory asset including deferred fuel and purchased power regulatory asset current	33,684	23,977
Palo Verde VIEs (Note 6)
Detail of regulatory assets
Regulatory asset including deferred fuel and purchased power regulatory asset current	20,812	20,933
Active Union Medical Trust
Detail of regulatory assets
Regulatory asset including deferred fuel and purchased power regulatory asset current	17,759	18,226
Power supply adjustor - interest
Detail of regulatory assets
Regulatory asset including deferred fuel and purchased power regulatory asset current	13,899	1,541
Navajo coal reclamation
Detail of regulatory assets
Regulatory asset including deferred fuel and purchased power regulatory asset current	11,628	13,862
Four Corners cost deferral
Detail of regulatory assets
Regulatory asset including deferred fuel and purchased power regulatory asset current	9,941	15,999
Mead-Phoenix transmission line contributions in aid of construction (“CIAC”)
Detail of regulatory assets
Regulatory asset including deferred fuel and purchased power regulatory asset current	8,799	9,048
Loss on reacquired debt
Detail of regulatory assets
Regulatory asset including deferred fuel and purchased power regulatory asset current	8,305	9,468
Tax expense adjustor mechanism (b) (e)
Detail of regulatory assets
Regulatory asset including deferred fuel and purchased power regulatory asset current	5,354	5,845
Lost fixed cost recovery
Detail of regulatory assets
Regulatory asset including deferred fuel and purchased power regulatory asset current	0	9,547
Other
Detail of regulatory assets
Regulatory asset including deferred fuel and purchased power regulatory asset current	$ 3,759	$ 6,630