Regulatory Matters - Schedule of Regulatory Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Detail of regulatory liabilities
Regulatory liabilities, current		$ 229,088	$ 234,912
Regulatory liabilities, non-current		2,450,169	2,267,835
Asset retirement obligations
Detail of regulatory liabilities
Regulatory liabilities, current		0	0
Regulatory liabilities, non-current		506,049	418,423
Other postretirement benefits
Detail of regulatory liabilities
Regulatory liabilities, current		37,705	37,575
Regulatory liabilities, non-current		349,588	139,634
Removal costs
Detail of regulatory liabilities
Regulatory liabilities, current		52,844	47,356
Regulatory liabilities, non-current		103,008	136,072
Income taxes — change in rates
Detail of regulatory liabilities
Regulatory liabilities, current		2,839	2,797
Regulatory liabilities, non-current		66,553	68,265
Four Corners coal reclamation
Detail of regulatory liabilities
Regulatory liabilities, current		5,460	1,059
Regulatory liabilities, non-current		49,435	51,704
Spent nuclear fuel
Detail of regulatory liabilities
Regulatory liabilities, current		6,768	6,676
Regulatory liabilities, non-current		44,221	51,019
Income taxes — deferred investment tax credit
Detail of regulatory liabilities
Regulatory liabilities, current		2,231	2,202
Regulatory liabilities, non-current		48,648	50,034
Renewable energy program
Detail of regulatory liabilities
Regulatory liabilities, current		39,442	39,287
Regulatory liabilities, non-current		103	10,300
Sundance maintenance
Detail of regulatory liabilities
Regulatory liabilities, current		2,989	5,698
Regulatory liabilities, non-current		11,508	11,319
Deferred property taxes
Detail of regulatory liabilities
Regulatory liabilities, current		0	0
Regulatory liabilities, non-current		13,856	7,046
Demand side management
Detail of regulatory liabilities
Regulatory liabilities, current		10,819	15,024
Regulatory liabilities, non-current		0	24,146
FERC transmission true up
Detail of regulatory liabilities
Regulatory liabilities, current		6,598	1,045
Regulatory liabilities, non-current		3,008	2,004
TCA balancing account
Detail of regulatory liabilities
Regulatory liabilities, current		2,902	0
Regulatory liabilities, non-current		4,672	0
Tax expense adjustor mechanism
Detail of regulatory liabilities
Regulatory liabilities, current		7,089	7,018
Regulatory liabilities, non-current		0	0
Tax expense adjustor mechanism | Forecast
Detail of regulatory liabilities
Regulatory liabilities, current	$ 7,000
Active union medical trust
Detail of regulatory liabilities
Regulatory liabilities, current		0	0
Regulatory liabilities, non-current		6,057	2,041
Deferred gains on utility property
Detail of regulatory liabilities
Regulatory liabilities, current		2,423	2,423
Regulatory liabilities, non-current		1,544	4,163
Other
Detail of regulatory liabilities
Regulatory liabilities, current		409	532
Regulatory liabilities, non-current		189	255
ACC | Excess deferred income taxes - ACC - Tax Cuts and Jobs Act
Detail of regulatory liabilities
Regulatory liabilities, current		41,330	59,918
Regulatory liabilities, non-current		1,012,583	1,054,053
FERC | Excess deferred income taxes - ACC - Tax Cuts and Jobs Act
Detail of regulatory liabilities
Regulatory liabilities, current		7,240	6,302
Regulatory liabilities, non-current		$ 229,147	$ 237,357