Regulatory Matters - Schedule of Regulatory Assets (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Detail of regulatory assets
Regulatory assets, current	$ 220,728	$ 106,875
Regulatory assets, non-current	1,415,091	1,313,428
Pension
Detail of regulatory assets
Regulatory assets, current	0	0
Regulatory assets, non-current	697,184	711,059
Retired power plant costs
Detail of regulatory assets
Regulatory assets, current	19,083	9,913
Regulatory assets, non-current	205,418	117,591
Income taxes — allowance for funds used during construction (AFUDC) equity
Detail of regulatory assets
Regulatory assets, current	6,202	6,305
Regulatory assets, non-current	158,356	152,118
Deferred fuel and purchased power — mark-to-market (Note 6)
Detail of regulatory assets
Regulatory assets, current	45,993	0
Regulatory assets, non-current	43,354	42,963
Deferred fuel and purchased power (b) (e)
Detail of regulatory assets
Regulatory assets, current	48,122	12,465
Regulatory assets, non-current	0	0
Four Corners cost deferral
Detail of regulatory assets
Regulatory assets, current	6,689	6,689
Regulatory assets, non-current	53,549	56,894
Income taxes — investment tax credit basis adjustment
Detail of regulatory assets
Regulatory assets, current	2,120	2,120
Regulatory assets, non-current	53,509	54,356
Lost fixed cost recovery (b)
Detail of regulatory assets
Regulatory assets, current	75,070	61,307
Regulatory assets, non-current	0	0
Palo Verde VIEs (Note 5)
Detail of regulatory assets
Regulatory assets, current	0	0
Regulatory assets, non-current	19,085	18,775
Deferred compensation
Detail of regulatory assets
Regulatory assets, current	0	0
Regulatory assets, non-current	37,161	35,595
Deferred property taxes
Detail of regulatory assets
Regulatory assets, current	0	0
Regulatory assets, non-current	85,694	73,200
Loss on reacquired debt
Detail of regulatory assets
Regulatory assets, current	1,637	1,637
Regulatory assets, non-current	16,124	16,942
Tax expense of Medicare subsidy
Detail of regulatory assets
Regulatory assets, current	1,503	1,513
Regulatory assets, non-current	9,922	10,589
Demand side management (c)
Detail of regulatory assets
Regulatory assets, current	5,122	3,744
Regulatory assets, non-current	0	0
AG-1 deferral
Detail of regulatory assets
Regulatory assets, current	0	0
Regulatory assets, non-current	10,058	5,868
Mead-Phoenix transmission line CIAC
Detail of regulatory assets
Regulatory assets, current	332	332
Regulatory assets, non-current	10,542	10,708
Transmission cost adjustor (b)
Detail of regulatory assets
Regulatory assets, current	8,115	0
Regulatory assets, non-current	0	1,588
Coal reclamation
Detail of regulatory assets
Regulatory assets, current	418	418
Regulatory assets, non-current	15,135	5,182
Other
Detail of regulatory assets
Regulatory assets, current	322	432
Regulatory assets, non-current	$ 0	$ 0