Regulatory Matters - Schedule of Regulatory Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2021	Sep. 30, 2021	Dec. 31, 2020
Detail of regulatory liabilities
Current		$ 379,193	$ 229,088
Non-Current		2,431,212	2,450,169
Asset retirement obligations
Detail of regulatory liabilities
Current		0	0
Non-Current		554,820	506,049
Other postretirement benefits
Detail of regulatory liabilities
Current		47,798	37,705
Non-Current		302,366	349,588
Deferred fuel and purchased power — mark-to-market (Note 7)
Detail of regulatory liabilities
Current		126,502	0
Non-Current		68,681	0
Removal costs
Detail of regulatory liabilities
Current		73,325	52,844
Non-Current		51,986	103,008
Income taxes — change in rates
Detail of regulatory liabilities
Current		2,839	2,839
Non-Current		63,707	66,553
Four Corners coal reclamation
Detail of regulatory liabilities
Current		5,460	5,460
Non-Current		50,151	49,435
Income taxes — deferred investment tax credit
Detail of regulatory liabilities
Current		2,231	2,231
Non-Current		46,431	48,648
Spent nuclear fuel
Detail of regulatory liabilities
Current		6,778	6,768
Non-Current		40,102	44,221
Renewable energy standard
Detail of regulatory liabilities
Current		32,193	39,442
Non-Current		115	103
FERC transmission true up
Detail of regulatory liabilities
Current		11,385	6,598
Non-Current		12,257	3,008
Property tax deferral
Detail of regulatory liabilities
Current		0	0
Non-Current		19,318	13,856
Sundance maintenance
Detail of regulatory liabilities
Current		0	2,989
Non-Current		13,271	11,508
Demand side management
Detail of regulatory liabilities
Current		7,701	10,819
Non-Current		3,713	0
Tax expense adjustor mechanism
Detail of regulatory liabilities
Current		7,398	7,089
Non-Current		0	0
Tax expense adjustor mechanism | Forecast
Detail of regulatory liabilities
Current	$ 7,000
Deferred gains on utility property
Detail of regulatory liabilities
Current		1,907	2,423
Non-Current		551	1,544
Active union medical trust
Detail of regulatory liabilities
Current		0	0
Non-Current		1,481	6,057
Other
Detail of regulatory liabilities
Current		5,018	3,311
Non-Current		107	4,861
ACC | Excess deferred income taxes - Tax Act
Detail of regulatory liabilities
Current		41,418	41,330
Non-Current		980,277	1,012,583
FERC | Excess deferred income taxes - Tax Act
Detail of regulatory liabilities
Current		7,240	7,240
Non-Current		$ 221,878	$ 229,147