Regulatory Matters - Schedule of Regulatory Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Detail of regulatory liabilities
Current	$ 308,019	$ 234,912
Non-Current	2,079,323	2,267,835
Asset retirement obligations
Detail of regulatory liabilities
Current	0	0
Non-Current	442,035	418,423
Removal costs
Detail of regulatory liabilities
Current	51,519	47,356
Non-Current	112,168	136,072
Other postretirement benefits
Detail of regulatory liabilities
Current	37,575	37,575
Non-Current	109,035	139,634
Four Corners coal reclamation
Detail of regulatory liabilities
Current	5,461	1,059
Non-Current	49,151	51,704
Spent nuclear fuel
Detail of regulatory liabilities
Current	6,520	6,676
Non-Current	46,526	51,019
Income taxes — change in rates
Detail of regulatory liabilities
Current	2,802	2,797
Non-Current	48,541	68,265
Income taxes — deferred investment tax credit
Detail of regulatory liabilities
Current	2,199	2,202
Non-Current	47,765	50,034
Renewable energy standard
Detail of regulatory liabilities
Current	35,813	39,287
Non-Current	1,128	10,300
Sundance maintenance
Detail of regulatory liabilities
Current	2,200	5,698
Non-Current	12,303	11,319
Demand side management
Detail of regulatory liabilities
Current	17,394	15,024
Non-Current	0	24,146
Property tax deferral
Detail of regulatory liabilities
Current	0	0
Non-Current	11,784	7,046
Deferred fuel and purchased power — mark-to-market (Note 7)
Detail of regulatory liabilities
Current	10,894	0
Non-Current	0	0
FERC transmission true up
Detail of regulatory liabilities
Current	5,965	1,045
Non-Current	4,291	2,004
Active union medical trust
Detail of regulatory liabilities
Current	0	0
Non-Current	6,993	2,041
Tax expense adjustor mechanism
Detail of regulatory liabilities
Current	6,542	7,018
Non-Current	0	0
Deferred gains on utility property
Detail of regulatory liabilities
Current	2,423	2,423
Non-Current	2,379	4,163
TCA balancing account
Detail of regulatory liabilities
Current	0	0
Non-Current	2,665	0
Other
Detail of regulatory liabilities
Current	250	532
Non-Current	275	255
ACC | Excess deferred income taxes - ACC - Tax Cuts and Jobs Act
Detail of regulatory liabilities
Current	113,206	59,918
Non-Current	952,531	1,054,053
United States Federal Energy Regulatory Commission | Excess deferred income taxes - ACC - Tax Cuts and Jobs Act
Detail of regulatory liabilities
Current	7,256	6,302
Non-Current	$ 229,753	$ 237,357