Regulatory Matters - Schedule of Regulatory Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Detail of regulatory assets
Regulatory assets, current	$ 291,713	$ 203,207
Regulatory assets, non-current	1,133,987	1,304,073
Pension
Detail of regulatory assets
Regulatory assets, current	0	0
Regulatory assets, non-current	469,953	660,223
Deferred fuel and purchased power
Detail of regulatory assets
Regulatory assets, current	175,835	70,137
Regulatory assets, non-current	0	0
Income taxes — AFUDC equity
Detail of regulatory assets
Regulatory assets, current	7,169	6,800
Regulatory assets, non-current	158,776	154,974
Retired power plant costs
Detail of regulatory assets
Regulatory assets, current	28,181	28,182
Regulatory assets, non-current	114,214	142,503
Ocotillo deferral
Detail of regulatory assets
Regulatory assets, current	0	0
Regulatory assets, non-current	95,723	38,144
SCR deferral
Detail of regulatory assets
Regulatory assets, current	0	0
Regulatory assets, non-current	81,307	52,644
Deferred property taxes
Detail of regulatory assets
Regulatory assets, current	8,569	8,569
Regulatory assets, non-current	49,626	58,196
Lost fixed cost recovery
Detail of regulatory assets
Regulatory assets, current	41,807	26,067
Regulatory assets, non-current	0	0
Deferred compensation
Detail of regulatory assets
Regulatory assets, current	0	0
Regulatory assets, non-current	36,195	36,464
Four Corners cost deferral
Detail of regulatory assets
Regulatory assets, current	8,077	8,077
Regulatory assets, non-current	24,075	32,152
Income taxes — investment tax credit basis adjustment
Detail of regulatory assets
Regulatory assets, current	1,113	1,098
Regulatory assets, non-current	24,291	24,981
Palo Verde VIE
Detail of regulatory assets
Regulatory assets, current	0	0
Regulatory assets, non-current	21,255	20,635
Coal reclamation
Detail of regulatory assets
Regulatory assets, current	1,068	1,546
Regulatory assets, non-current	16,999	17,688
Deferred fuel and purchased power - mark-to-market
Detail of regulatory assets
Regulatory assets, current	3,341	36,887
Regulatory assets, non-current	9,244	33,185
Loss on reacquired debt
Detail of regulatory assets
Regulatory assets, current	1,689	1,637
Regulatory assets, non-current	10,877	12,031
Mead-Phoenix transmission line — contributions in aid of construction
Detail of regulatory assets
Regulatory assets, current	332	332
Regulatory assets, non-current	9,380	9,712
Demand side management
Detail of regulatory assets
Regulatory assets, current	0	0
Regulatory assets, non-current	7,268	0
Tax expense adjustor mechanism
Detail of regulatory assets
Regulatory assets, current	6,226	1,612
Regulatory assets, non-current	0	0
Tax expense of Medicare subsidy
Detail of regulatory assets
Regulatory assets, current	1,235	1,235
Regulatory assets, non-current	3,704	4,940
PSA interest
Detail of regulatory assets
Regulatory assets, current	4,355	1,917
Regulatory assets, non-current	0	0
TCA balancing account
Detail of regulatory assets
Regulatory assets, current	0	6,324
Regulatory assets, non-current	0	2,885
Other
Detail of regulatory assets
Regulatory assets, current	2,716	2,787
Regulatory assets, non-current	$ 1,100	$ 2,716