Regulatory Matters - Schedule of Regulatory Assets (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Detail of regulatory assets
Regulatory assets, current	$ 258,317	$ 106,875
Regulatory assets, non-current	1,381,179	1,313,428
Pension
Detail of regulatory assets
Regulatory assets, current	0	0
Regulatory assets, non-current	686,511	711,059
Retired power plant costs
Detail of regulatory assets
Regulatory assets, current	28,647	9,913
Regulatory assets, non-current	194,639	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	170,622	152,118
Deferred fuel and purchased power — mark-to-market (Note 6)
Detail of regulatory assets
Regulatory assets, current	45,463	0
Regulatory assets, non-current	33,115	42,963
Deferred fuel and purchased power (b) (d)
Detail of regulatory assets
Regulatory assets, current	73,966	12,465
Regulatory assets, non-current	0	0
Four Corners cost deferral
Detail of regulatory assets
Regulatory assets, current	8,077	6,689
Regulatory assets, non-current	50,324	56,894
Income taxes — investment tax credit basis adjustment
Detail of regulatory assets
Regulatory assets, current	2,120	2,120
Regulatory assets, non-current	53,225	54,356
Lost fixed cost recovery (b)
Detail of regulatory assets
Regulatory assets, current	67,500	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	19,240	18,775
Deferred compensation
Detail of regulatory assets
Regulatory assets, current	0	0
Regulatory assets, non-current	37,265	35,595
Deferred property taxes
Detail of regulatory assets
Regulatory assets, current	8,569	0
Regulatory assets, non-current	77,408	73,200
Loss on reacquired debt
Detail of regulatory assets
Regulatory assets, current	1,637	1,637
Regulatory assets, non-current	15,715	16,942
Tax expense of Medicare subsidy
Detail of regulatory assets
Regulatory assets, current	1,503	1,513
Regulatory assets, non-current	9,074	10,589
Demand side management (c)
Detail of regulatory assets
Regulatory assets, current	0	3,744
Regulatory assets, non-current	0	0
AG-1 deferral
Detail of regulatory assets
Regulatory assets, current	2,654	0
Regulatory assets, non-current	9,136	5,868
Mead-Phoenix transmission line CIAC
Detail of regulatory assets
Regulatory assets, current	332	332
Regulatory assets, non-current	10,459	10,708
Transmission cost adjustor (b)
Detail of regulatory assets
Regulatory assets, current	4,345	0
Regulatory assets, non-current	0	1,588
Coal reclamation
Detail of regulatory assets
Regulatory assets, current	1,068	418
Regulatory assets, non-current	14,446	5,182
Other
Detail of regulatory assets
Regulatory assets, current	6,234	432
Regulatory assets, non-current	$ 0	$ 0