Regulatory Matters - Schedule of Regulatory Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Detail of regulatory assets
Regulatory assets, current	$ 106,875	$ 149,555
Regulatory assets, non-current	1,313,428	1,214,146
Pension
Detail of regulatory assets
Regulatory assets, current	0	0
Regulatory assets, non-current	711,059	619,223
Retired power plant costs
Detail of regulatory assets
Regulatory assets, current	9,913	9,913
Regulatory assets, non-current	117,591	127,518
Income taxes - AFUDC equity
Detail of regulatory assets
Regulatory assets, current	6,305	5,495
Regulatory assets, non-current	152,118	133,712
Deferred fuel and purchased power - mark-to-market
Detail of regulatory assets
Regulatory assets, current	0	71,852
Regulatory assets, non-current	42,963	69,697
Four Corners cost deferral
Detail of regulatory assets
Regulatory assets, current	6,689	6,689
Regulatory assets, non-current	56,894	63,582
Income taxes — investment tax credit basis adjustment
Detail of regulatory assets
Regulatory assets, current	2,120	1,766
Regulatory assets, non-current	54,356	48,462
Lost fixed cost recovery
Detail of regulatory assets
Regulatory assets, current	61,307	45,507
Regulatory assets, non-current	0	0
Palo Verde VIE
Detail of regulatory assets
Regulatory assets, current	0	0
Regulatory assets, non-current	18,775	18,143
Deferred compensation
Detail of regulatory assets
Regulatory assets, current	0	0
Regulatory assets, non-current	35,595	34,751
Deferred property taxes
Detail of regulatory assets
Regulatory assets, current	0	0
Regulatory assets, non-current	73,200	50,453
Loss on reacquired debt
Detail of regulatory assets
Regulatory assets, current	1,637	1,515
Regulatory assets, non-current	16,942	16,375
AG-1 deferral
Detail of regulatory assets
Regulatory assets, current	0	0
Regulatory assets, non-current	5,868	0
Demand side management (b)
Detail of regulatory assets
Regulatory assets, current	3,744	0
Regulatory assets, non-current	0	0
Tax expense of Medicare subsidy
Detail of regulatory assets
Regulatory assets, current	1,513	1,520
Regulatory assets, non-current	10,589	12,163
Transmission vegetation management
Detail of regulatory assets
Regulatory assets, current	0	4,543
Regulatory assets, non-current	0	0
Mead-Phoenix transmission line CIAC
Detail of regulatory assets
Regulatory assets, current	332	332
Regulatory assets, non-current	10,708	11,040
Deferred fuel and purchased power
Detail of regulatory assets
Regulatory assets, current	12,465	0
Regulatory assets, non-current	0	0
Coal reclamation
Detail of regulatory assets
Regulatory assets, current	418	418
Regulatory assets, non-current	5,182	6,085
Other
Detail of regulatory assets
Regulatory assets, current	432	5
Regulatory assets, non-current	$ 1,588	$ 2,942