Regulatory Matters - Schedule of Regulatory Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Detail of regulatory assets
Regulatory assets, current	$ 203,207	$ 166,902
Regulatory assets, non-current	1,304,073	1,342,941
Pension
Detail of regulatory assets
Regulatory assets, current	0	0
Regulatory assets, non-current	660,223	733,351
Retired power plant costs
Detail of regulatory assets
Regulatory assets, current	28,182	28,182
Regulatory assets, non-current	142,503	167,164
Income taxes - AFUDC equity
Detail of regulatory assets
Regulatory assets, current	6,800	6,457
Regulatory assets, non-current	154,974	151,467
Deferred fuel and purchased power
Detail of regulatory assets
Regulatory assets, current	70,137	37,164
Regulatory assets, non-current	0	0
Deferred fuel and purchased power - mark-to-market
Detail of regulatory assets
Regulatory assets, current	36,887	31,728
Regulatory assets, non-current	33,185	23,768
Property tax deferral
Detail of regulatory assets
Regulatory assets, current	8,569	8,569
Regulatory assets, non-current	58,196	66,356
SCR deferral
Detail of regulatory assets
Regulatory assets, current	0	0
Regulatory assets, non-current	52,644	23,276
Four Corners cost deferral
Detail of regulatory assets
Regulatory assets, current	8,077	8,077
Regulatory assets, non-current	32,152	40,228
Lost fixed cost recovery (b)
Detail of regulatory assets
Regulatory assets, current	0	0
Regulatory assets, non-current	38,144	0
Deferred compensation
Detail of regulatory assets
Regulatory assets, current	0	0
Regulatory assets, non-current	36,464	36,523
Income taxes — investment tax credit basis adjustment
Detail of regulatory assets
Regulatory assets, current	1,098	1,079
Regulatory assets, non-current	24,981	25,522
Lost fixed cost recovery
Detail of regulatory assets
Regulatory assets, current	26,067	32,435
Regulatory assets, non-current	0	0
Palo Verde VIE
Detail of regulatory assets
Regulatory assets, current	0	0
Regulatory assets, non-current	20,635	20,015
Coal reclamation
Detail of regulatory assets
Regulatory assets, current	1,546	1,546
Regulatory assets, non-current	17,688	15,607
Loss on reacquired debt
Detail of regulatory assets
Regulatory assets, current	1,637	1,637
Regulatory assets, non-current	12,031	13,668
Mead-Phoenix transmission line - contributions in aid of construction
Detail of regulatory assets
Regulatory assets, current	332	332
Regulatory assets, non-current	9,712	10,044
TCA balancing account
Detail of regulatory assets
Regulatory assets, current	6,324	3,860
Regulatory assets, non-current	2,885	772
Tax expense of Medicare subsidy
Detail of regulatory assets
Regulatory assets, current	1,235	1,235
Regulatory assets, non-current	4,940	6,176
AG-1 deferral
Detail of regulatory assets
Regulatory assets, current	2,787	2,654
Regulatory assets, non-current	2,716	5,819
Tax expense adjustor mechanism
Detail of regulatory assets
Regulatory assets, current	1,612	0
Regulatory assets, non-current	0	0
Other
Detail of regulatory assets
Regulatory assets, current	1,917	1,947
Regulatory assets, non-current	$ 0	$ 3,185