Regulatory Matters - Schedule of Regulatory Liabilities (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Detail of regulatory liabilities
Current	$ 279,479	$ 234,912
Non-Current	2,089,987	2,267,835
Asset retirement obligations
Detail of regulatory liabilities
Current	0	0
Non-Current	408,126	418,423
Removal costs
Detail of regulatory liabilities
Current	47,300	47,356
Non-Current	126,036	136,072
Other postretirement benefits
Detail of regulatory liabilities
Current	37,575	37,575
Non-Current	118,398	139,634
Four Corners coal reclamation
Detail of regulatory liabilities
Current	5,461	1,059
Non-Current	48,795	51,704
Spent nuclear fuel
Detail of regulatory liabilities
Current	6,068	6,676
Non-Current	47,901	51,019
Income taxes — change in rates
Detail of regulatory liabilities
Current	2,802	2,797
Non-Current	50,163	68,265
Income taxes — deferred investment tax credit
Detail of regulatory liabilities
Current	2,202	2,202
Non-Current	49,133	50,034
Renewable energy standard
Detail of regulatory liabilities
Current	38,934	39,287
Non-Current	22	10,300
Sundance maintenance
Detail of regulatory liabilities
Current	1,100	5,698
Non-Current	13,001	11,319
Demand side management
Detail of regulatory liabilities
Current	3,068	15,024
Non-Current	6,138	24,146
Property tax deferral
Detail of regulatory liabilities
Current	0	0
Non-Current	8,603	7,046
FERC transmission true up
Detail of regulatory liabilities
Current	5,452	1,045
Non-Current	2,209	2,004
Active union medical trust
Detail of regulatory liabilities
Current	0	0
Non-Current	7,629	2,041
Tax expense adjustor mechanism
Detail of regulatory liabilities
Current	6,450	7,018
Non-Current	0	0
Deferred gains on utility property
Detail of regulatory liabilities
Current	2,423	2,423
Non-Current	2,973	4,163
Other
Detail of regulatory liabilities
Current	220	532
Non-Current	331	255
ACC | Excess deferred income taxes - ACC - Tax Cuts and Jobs Act
Detail of regulatory liabilities
Current	113,168	59,918
Non-Current	966,576	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	$ 233,953	$ 237,357