Regulatory Matters - Schedule of Regulatory Assets (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Detail of regulatory assets
Current	$ 225,471	$ 203,207
Non-Current	1,302,448	1,304,073
Pension
Detail of regulatory assets
Current	0	0
Non-Current	652,691	660,223
Retired power plant costs
Detail of regulatory assets
Current	28,182	28,182
Non-Current	135,349	142,503
Income taxes — allowance for funds used during construction (AFUDC) equity
Detail of regulatory assets
Current	6,815	6,800
Non-Current	155,369	154,974
Deferred fuel and purchased power — mark-to-market (Note 7)
Detail of regulatory assets
Current	51,954	36,887
Non-Current	32,576	33,185
Deferred fuel and purchased power
Detail of regulatory assets
Current	77,730	70,137
Non-Current	0	0
Deferred property taxes
Detail of regulatory assets
Current	8,569	8,569
Non-Current	56,053	58,196
SCR deferral
Detail of regulatory assets
Current	0	0
Non-Current	58,258	52,644
Ocotillo deferral
Detail of regulatory assets
Current	0	0
Non-Current	51,767	38,144
Four Corners cost deferral
Detail of regulatory assets
Current	8,077	8,077
Non-Current	30,133	32,152
Deferred compensation
Detail of regulatory assets
Current	0	0
Non-Current	37,550	36,464
Lost fixed cost recovery
Detail of regulatory assets
Current	28,885	26,067
Non-Current	0	0
Income taxes — investment tax credit basis adjustment
Detail of regulatory assets
Current	1,098	1,098
Non-Current	24,920	24,981
Palo Verde VIEs (Note 6)
Detail of regulatory assets
Current	0	0
Non-Current	20,790	20,635
Coal reclamation
Detail of regulatory assets
Current	1,068	1,546
Non-Current	17,800	17,688
Loss on reacquired debt
Detail of regulatory assets
Current	1,637	1,637
Non-Current	11,636	12,031
Mead-Phoenix transmission line contributions in aid of construction (CIAC)
Detail of regulatory assets
Current	332	332
Non-Current	9,629	9,712
TCA balancing account
Detail of regulatory assets
Current	6,048	6,324
Non-Current	1,027	2,885
Tax expense of Medicare subsidy
Detail of regulatory assets
Current	1,238	1,235
Non-Current	4,881	4,940
AG-1 deferral
Detail of regulatory assets
Current	2,787	2,787
Non-Current	2,019	2,716
Tax expense adjustor mechanism
Detail of regulatory assets
Current	942	1,612
Non-Current	0	0
Other
Detail of regulatory assets
Current	109	1,917
Non-Current	$ 0	$ 0