Regulatory Matters - Schedule of Regulatory Assets (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Detail of regulatory assets
Current	$ 272,870	$ 203,207
Non-Current	1,305,437	1,304,073
Pension
Detail of regulatory assets
Current	0	0
Non-Current	656,092	660,223
Income taxes — allowance for funds used during construction ("AFUDC") equity
Detail of regulatory assets
Current	6,815	6,800
Non-Current	159,005	154,974
Retired power plant costs
Detail of regulatory assets
Current	28,182	28,182
Non-Current	121,259	142,503
Deferred fuel and purchased power
Detail of regulatory assets
Current	162,111	70,137
Non-Current	0	0
Ocotillo deferral
Detail of regulatory assets
Current	0	0
Non-Current	80,359	38,144
SCR deferral
Detail of regulatory assets
Current	0	0
Non-Current	74,576	52,644
Deferred property taxes
Detail of regulatory assets
Current	8,569	8,569
Non-Current	51,769	58,196
Lost fixed cost recovery
Detail of regulatory assets
Current	37,868	26,067
Non-Current	0	0
Deferred compensation
Detail of regulatory assets
Current	0	0
Non-Current	36,481	36,464
Four Corners cost deferral
Detail of regulatory assets
Current	8,077	8,077
Non-Current	26,094	32,152
Income taxes — investment tax credit basis adjustment
Detail of regulatory assets
Current	1,097	1,098
Non-Current	23,850	24,981
Palo Verde VIEs (Note 6)
Detail of regulatory assets
Current	0	0
Non-Current	21,100	20,635
Coal reclamation
Detail of regulatory assets
Current	1,068	1,546
Non-Current	17,266	17,688
Loss on reacquired debt
Detail of regulatory assets
Current	1,637	1,637
Non-Current	10,846	12,031
Mead-Phoenix transmission line contributions in aid of construction ("CIAC")
Detail of regulatory assets
Current	332	332
Non-Current	9,463	9,712
Demand side management
Detail of regulatory assets
Current	0	0
Non-Current	7,259	0
Tax expense adjustor mechanism
Detail of regulatory assets
Current	6,121	1,612
Non-Current	0	0
Deferred fuel and purchased power — mark-to-market (Note 7)
Detail of regulatory assets
Current	0	36,887
Non-Current	5,664	33,185
Tax expense of Medicare subsidy
Detail of regulatory assets
Current	1,238	1,235
Non-Current	3,728	4,940
AG-1 deferral
Detail of regulatory assets
Current	2,787	2,787
Non-Current	626	2,716
TCA balancing account
Detail of regulatory assets
Current	4,490	6,324
Non-Current	0	2,885
Other
Detail of regulatory assets
Current	2,478	1,917
Non-Current	$ 0	$ 0