Regulatory Matters - Schedule of Regulatory Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Detail of regulatory assets
Regulatory assets, current	$ 149,555	$ 136,734
Regulatory assets, non-current	1,214,146	1,054,087
Pension
Detail of regulatory assets
Regulatory assets, current	0	0
Regulatory assets, non-current	619,223	485,037
Retired power plant costs
Detail of regulatory assets
Regulatory assets, current	9,913	9,913
Regulatory assets, non-current	127,518	136,182
Income taxes - AFUDC equity
Detail of regulatory assets
Regulatory assets, current	5,495	4,813
Regulatory assets, non-current	133,712	118,396
Deferred fuel and purchased power - mark-to-market
Detail of regulatory assets
Regulatory assets, current	71,852	51,209
Regulatory assets, non-current	69,697	46,233
Four Corners cost deferral
Detail of regulatory assets
Regulatory assets, current	6,689	6,689
Regulatory assets, non-current	63,582	70,565
Income taxes — investment tax credit basis adjustment
Detail of regulatory assets
Regulatory assets, current	1,766	1,716
Regulatory assets, non-current	48,462	46,200
Lost fixed cost recovery
Detail of regulatory assets
Regulatory assets, current	45,507	37,612
Regulatory assets, non-current	0	0
Palo Verde VIE
Detail of regulatory assets
Regulatory assets, current	0	0
Regulatory assets, non-current	18,143	34,440
Deferred compensation
Detail of regulatory assets
Regulatory assets, current	0	0
Regulatory assets, non-current	34,751	34,162
Deferred property taxes
Detail of regulatory assets
Regulatory assets, current	0	0
Regulatory assets, non-current	50,453	30,283
Loss on reacquired debt
Detail of regulatory assets
Regulatory assets, current	1,515	1,435
Regulatory assets, non-current	16,375	16,410
Tax expense of Medicare subsidy
Detail of regulatory assets
Regulatory assets, current	1,520	1,528
Regulatory assets, non-current	12,163	13,756
Transmission vegetation management
Detail of regulatory assets
Regulatory assets, current	4,543	9,086
Regulatory assets, non-current	0	4,543
Mead-Phoenix transmission line CIAC
Detail of regulatory assets
Regulatory assets, current	332	332
Regulatory assets, non-current	11,040	11,372
Deferred fuel and purchased power
Detail of regulatory assets
Regulatory assets, current	0	6,926
Regulatory assets, non-current	0	0
Coal reclamation
Detail of regulatory assets
Regulatory assets, current	418	418
Regulatory assets, non-current	6,085	6,503
Pension and other postretirement benefits deferral
Detail of regulatory assets
Regulatory assets, current	0	4,238
Regulatory assets, non-current	0	0
Other
Detail of regulatory assets
Regulatory assets, current	5	819
Regulatory assets, non-current	$ 2,942	$ 5