Regulatory Matters - Schedule of Regulatory Assets (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Detail of regulatory assets
Regulatory assets, current	$ 229,559	$ 248,088
Regulatory assets, non-current	1,233,062	1,202,302
Pension
Detail of regulatory assets
Regulatory assets, current	0	0
Regulatory assets, non-current	604,556	576,188
Retired power plant costs
Detail of regulatory assets
Regulatory assets, current	25,934	27,402
Regulatory assets, non-current	173,877	188,843
Income taxes — allowance for funds used during construction (AFUDC) equity
Detail of regulatory assets
Regulatory assets, current	5,882	3,828
Regulatory assets, non-current	144,104	142,852
Deferred fuel and purchased power — mark-to-market (Note 7)
Detail of regulatory assets
Regulatory assets, current	42,684	52,100
Regulatory assets, non-current	45,047	34,845
Deferred fuel and purchased power
Detail of regulatory assets
Regulatory assets, current	74,898	75,637
Regulatory assets, non-current	0	0
Four Corners cost deferral
Detail of regulatory assets
Regulatory assets, current	8,077	8,077
Regulatory assets, non-current	44,267	48,305
Income taxes — investment tax credit basis adjustment
Detail of regulatory assets
Regulatory assets, current	1,066	1,066
Regulatory assets, non-current	25,850	26,218
Lost fixed cost recovery
Detail of regulatory assets
Regulatory assets, current	48,484	59,844
Regulatory assets, non-current	0	0
Palo Verde VIEs (Note 6)
Detail of regulatory assets
Regulatory assets, current	0	0
Regulatory assets, non-current	19,705	19,395
Deferred compensation
Detail of regulatory assets
Regulatory assets, current	0	0
Regulatory assets, non-current	37,750	36,413
Deferred property taxes
Detail of regulatory assets
Regulatory assets, current	8,569	8,569
Regulatory assets, non-current	71,562	74,926
Loss on reacquired debt
Detail of regulatory assets
Regulatory assets, current	1,637	1,637
Regulatory assets, non-current	14,487	15,305
Tax expense of Medicare subsidy
Detail of regulatory assets
Regulatory assets, current	1,235	1,236
Regulatory assets, non-current	6,984	7,415
TCA balancing account
Detail of regulatory assets
Regulatory assets, current	6,110	1,220
Regulatory assets, non-current	0	0
AG-1 deferral
Detail of regulatory assets
Regulatory assets, current	2,654	2,654
Regulatory assets, non-current	7,146	8,472
Mead-Phoenix transmission line CIAC
Detail of regulatory assets
Regulatory assets, current	332	332
Regulatory assets, non-current	10,210	10,376
Coal reclamation
Detail of regulatory assets
Regulatory assets, current	1,546	1,068
Regulatory assets, non-current	11,842	12,396
SCR deferral
Detail of regulatory assets
Regulatory assets, current	0	0
Regulatory assets, non-current	9,482	353
Other
Detail of regulatory assets
Regulatory assets, current	451	3,418
Regulatory assets, non-current	$ 6,193	$ 0