Regulatory Matters - Schedule of Regulatory Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2022	Mar. 31, 2021	Dec. 31, 2020
Detail of regulatory liabilities
Current		$ 250,228	$ 229,088
Non-Current		2,427,769	2,450,169
Asset retirement obligations
Detail of regulatory liabilities
Current		0	0
Non-Current		519,015	506,049
Other postretirement benefits
Detail of regulatory liabilities
Current		37,705	37,705
Non-Current		337,853	349,588
Removal costs
Detail of regulatory liabilities
Current		55,247	52,844
Non-Current		89,937	103,008
Income taxes — change in rates
Detail of regulatory liabilities
Current		2,839	2,839
Non-Current		66,374	66,553
Four Corners coal reclamation
Detail of regulatory liabilities
Current		5,461	5,460
Non-Current		49,703	49,435
Income taxes — deferred investment tax credit
Detail of regulatory liabilities
Current		2,231	2,231
Non-Current		48,507	48,648
Spent nuclear fuel
Detail of regulatory liabilities
Current		6,831	6,768
Non-Current		43,059	44,221
Renewable energy standard
Detail of regulatory liabilities
Current		34,460	39,442
Non-Current		30	103
Deferred fuel and purchased power — mark-to-market (Note 7)
Detail of regulatory liabilities
Current		20,829	0
Non-Current		0	0
Property tax deferral
Detail of regulatory liabilities
Current		0	0
Non-Current		15,022	13,856
Sundance maintenance
Detail of regulatory liabilities
Current		2,867	2,989
Non-Current		11,910	11,508
Demand side management
Detail of regulatory liabilities
Current		7,821	10,819
Non-Current		5,975	0
FERC transmission true up
Detail of regulatory liabilities
Current		7,630	6,598
Non-Current		2,379	3,008
TCA balancing account
Detail of regulatory liabilities
Current		7,315	2,902
Non-Current		1,754	4,672
Tax expense adjustor mechanism
Detail of regulatory liabilities
Current		7,452	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		939	1,544
Active union medical trust
Detail of regulatory liabilities
Current		0	0
Non-Current		2,337	6,057
Other
Detail of regulatory liabilities
Current		524	409
Non-Current		59	189
ACC | Excess deferred income taxes - ACC - Tax Cuts and Jobs Act
Detail of regulatory liabilities
Current		41,353	41,330
Non-Current		1,004,226	1,012,583
United States Federal Energy Regulatory Commission | Excess deferred income taxes - ACC - Tax Cuts and Jobs Act
Detail of regulatory liabilities
Current		7,240	7,240
Non-Current		$ 228,690	$ 229,147