Regulatory Matters - Schedule of Regulatory Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2021	Jun. 30, 2021	Dec. 31, 2020
Detail of regulatory liabilities
Current		$ 327,612	$ 229,088
Non-Current		2,443,312	2,450,169
Asset retirement obligations
Detail of regulatory liabilities
Current		0	0
Non-Current		567,900	506,049
Other postretirement benefits
Detail of regulatory liabilities
Current		47,798	37,705
Non-Current		314,218	349,588
Removal costs
Detail of regulatory liabilities
Current		69,348	52,844
Non-Current		61,601	103,008
Deferred fuel and purchased power — mark-to-market (Note 7)
Detail of regulatory liabilities
Current		82,082	0
Non-Current		27,305	0
Income taxes — change in rates
Detail of regulatory liabilities
Current		2,839	2,839
Non-Current		65,319	66,553
Four Corners coal reclamation
Detail of regulatory liabilities
Current		5,461	5,460
Non-Current		49,904	49,435
Income taxes — deferred investment tax credit
Detail of regulatory liabilities
Current		2,231	2,231
Non-Current		47,677	48,648
Spent nuclear fuel
Detail of regulatory liabilities
Current		6,510	6,768
Non-Current		41,815	44,221
Renewable energy standard
Detail of regulatory liabilities
Current		30,665	39,442
Non-Current		0	103
Property tax deferral
Detail of regulatory liabilities
Current		0	0
Non-Current		16,188	13,856
Demand side management
Detail of regulatory liabilities
Current		3,149	10,819
Non-Current		12,457	0
Sundance maintenance
Detail of regulatory liabilities
Current		556	2,989
Non-Current		12,312	11,508
FERC transmission true up
Detail of regulatory liabilities
Current		7,547	6,598
Non-Current		3,511	3,008
TCA balancing account
Detail of regulatory liabilities
Current		10,750	2,902
Non-Current		159	4,672
Tax expense adjustor mechanism
Detail of regulatory liabilities
Current		7,148	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		2,423	2,423
Non-Current		333	1,544
Active union medical trust
Detail of regulatory liabilities
Current		0	0
Non-Current		2,347	6,057
Other
Detail of regulatory liabilities
Current		484	409
Non-Current		289	189
ACC | Excess deferred income taxes - Tax Act
Detail of regulatory liabilities
Current		41,381	41,330
Non-Current		993,982	1,012,583
FERC | Excess deferred income taxes - Tax Act
Detail of regulatory liabilities
Current		7,240	7,240
Non-Current		$ 225,995	$ 229,147