Regulatory Matters - Schedule of Regulatory Assets (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Detail of regulatory assets
Current	$ 184,459	$ 166,902
Non-Current	1,321,431	1,342,941
Pension
Detail of regulatory assets
Current	0	0
Non-Current	712,907	733,351
Retired power plant costs
Detail of regulatory assets
Current	28,182	28,182
Non-Current	153,122	167,164
Income taxes — allowance for funds used during construction (AFUDC) equity
Detail of regulatory assets
Current	6,457	6,457
Non-Current	152,388	151,467
Deferred fuel and purchased power — mark-to-market (Note 7)
Detail of regulatory assets
Current	55,729	31,728
Non-Current	21,516	23,768
Deferred fuel and purchased power
Detail of regulatory assets
Current	30,559	37,164
Non-Current	0	0
Four Corners cost deferral
Detail of regulatory assets
Current	8,077	8,077
Non-Current	36,190	40,228
Income taxes — investment tax credit basis adjustment
Detail of regulatory assets
Current	1,079	1,079
Non-Current	25,170	25,522
Lost fixed cost recovery
Detail of regulatory assets
Current	25,746	32,435
Non-Current	0	0
Palo Verde VIEs (Note 6)
Detail of regulatory assets
Current	0	0
Non-Current	20,325	20,015
Deferred compensation
Detail of regulatory assets
Current	0	0
Non-Current	37,572	36,523
Deferred property taxes
Detail of regulatory assets
Current	8,569	8,569
Non-Current	62,072	66,356
Loss on reacquired debt
Detail of regulatory assets
Current	1,637	1,637
Non-Current	12,850	13,668
Tax expense of Medicare subsidy
Detail of regulatory assets
Current	1,235	1,235
Non-Current	5,772	6,176
TCA balancing account
Detail of regulatory assets
Current	5,381	3,860
Non-Current	3,895	772
AG-1 deferral
Detail of regulatory assets
Current	2,787	2,654
Non-Current	4,110	5,819
Mead-Phoenix transmission line CIAC
Detail of regulatory assets
Current	332	332
Non-Current	9,878	10,044
Coal reclamation
Detail of regulatory assets
Current	1,546	1,546
Non-Current	16,250	15,607
SCR deferral
Detail of regulatory assets
Current	0	0
Non-Current	37,919	23,276
Tax expense adjustor mechanism
Detail of regulatory assets
Current	3,149	0
Non-Current	0	0
Ocotillo deferral
Detail of regulatory assets
Current	0	0
Non-Current	9,495	0
Other
Detail of regulatory assets
Current	3,994	1,947
Non-Current	$ 0	$ 3,185