Regulatory Matters - Schedule of Regulatory Assets (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Detail of regulatory assets
Regulatory assets, current	$ 168,753	$ 149,555
Regulatory assets, non-current	1,203,474	1,214,146
Pension
Detail of regulatory assets
Regulatory assets, current	0	0
Regulatory assets, non-current	610,569	619,223
Retired power plant costs
Detail of regulatory assets
Regulatory assets, current	9,913	9,913
Regulatory assets, non-current	125,037	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	132,149	133,712
Deferred fuel and purchased power — mark-to-market (Note 6)
Detail of regulatory assets
Regulatory assets, current	86,160	71,852
Regulatory assets, non-current	69,708	69,697
Four Corners cost deferral (b)
Detail of regulatory assets
Regulatory assets, current	6,689	6,689
Regulatory assets, non-current	61,910	63,582
Income taxes — investment tax credit basis adjustment
Detail of regulatory assets
Regulatory assets, current	1,852	1,766
Regulatory assets, non-current	48,347	48,462
Lost fixed cost recovery (b)
Detail of regulatory assets
Regulatory assets, current	48,702	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	18,311	18,143
Deferred compensation
Detail of regulatory assets
Regulatory assets, current	0	0
Regulatory assets, non-current	35,871	34,751
Deferred property taxes
Detail of regulatory assets
Regulatory assets, current	0	0
Regulatory assets, non-current	56,589	50,453
Loss on reacquired debt
Detail of regulatory assets
Regulatory assets, current	1,515	1,515
Regulatory assets, non-current	15,996	16,375
Tax expense of Medicare subsidy
Detail of regulatory assets
Regulatory assets, current	1,512	1,520
Regulatory assets, non-current	12,073	12,163
Transmission vegetation management
Detail of regulatory assets
Regulatory assets, current	2,272	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	10,957	11,040
Transmission cost adjustor (b)
Detail of regulatory assets
Regulatory assets, current	3,969	0
Regulatory assets, non-current	462	2,942
Coal reclamation
Detail of regulatory assets
Regulatory assets, current	418	418
Regulatory assets, non-current	5,495	6,085
Other
Detail of regulatory assets
Regulatory assets, current	0	5
Regulatory assets, non-current	$ 0	$ 0