Regulatory Matters - Schedule of Regulatory Assets (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Detail of regulatory assets
Regulatory assets, current	$ 122,220	$ 149,555
Regulatory assets, non-current	1,212,150	1,214,146
Pension
Detail of regulatory assets
Regulatory assets, current	0	0
Regulatory assets, non-current	608,312	619,223
Retired power plant costs
Detail of regulatory assets
Regulatory assets, current	9,913	9,913
Regulatory assets, non-current	120,072	127,518
Income taxes — allowance for funds used during construction (AFUDC) equity
Detail of regulatory assets
Regulatory assets, current	5,419	5,495
Regulatory assets, non-current	151,451	133,712
Deferred fuel and purchased power — mark-to-market (Note 6)
Detail of regulatory assets
Regulatory assets, current	30,748	71,852
Regulatory assets, non-current	53,350	69,697
Deferred fuel and purchased power (c) (e)
Detail of regulatory assets
Regulatory assets, current	8,132	0
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	58,566	63,582
Income taxes — investment tax credit basis adjustment
Detail of regulatory assets
Regulatory assets, current	1,852	1,766
Regulatory assets, non-current	46,699	48,462
Lost fixed cost recovery (b)
Detail of regulatory assets
Regulatory assets, current	55,297	45,507
Regulatory assets, non-current	0	0
Palo Verde VIEs (Note 5)
Detail of regulatory assets
Regulatory assets, current	0	0
Regulatory assets, non-current	18,620	18,143
Deferred compensation
Detail of regulatory assets
Regulatory assets, current	0	0
Regulatory assets, non-current	36,071	34,751
Deferred property taxes
Detail of regulatory assets
Regulatory assets, current	0	0
Regulatory assets, non-current	67,547	50,453
Loss on reacquired debt
Detail of regulatory assets
Regulatory assets, current	1,592	1,515
Regulatory assets, non-current	16,521	16,375
Tax expense of Medicare subsidy
Detail of regulatory assets
Regulatory assets, current	1,512	1,520
Regulatory assets, non-current	10,774	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	10,791	11,040
Transmission cost adjustor (b)
Detail of regulatory assets
Regulatory assets, current	0	0
Regulatory assets, non-current	4,687	2,942
Coal reclamation
Detail of regulatory assets
Regulatory assets, current	418	418
Regulatory assets, non-current	5,286	6,085
Other
Detail of regulatory assets
Regulatory assets, current	316	5
Regulatory assets, non-current	$ 3,403	$ 0