Long-Term Debt and Capital Leases Schedule of Long-Term Debt (Details) - USD ($) $ in Thousands	Dec. 31, 2024	Mar. 31, 2023
Montana First Mortgage Bonds Due 2024
Debt Instrument [Line Items]
Debt Instrument, Face Amount	$ 100,000
Secured Debt Montana 3.88% Due 2028
Debt Instrument [Line Items]
Debt Instrument, Face Amount	$ 144,700
Interest rate, stated percentage	3.88%
Long-term Debt, Maturity Date	Jul. 01, 2028
Montana 2.00% Due 2023
Debt Instrument [Line Items]
Debt Instrument, Face Amount	$ 144,700
Interest rate, stated percentage	2.00%
South Dakota First Mortgage Bonds Due May 1, 2033
Debt Instrument [Line Items]
Interest rate, stated percentage	5.42%
Long-term Debt, Maturity Date	May 01, 2033
Montana First Mortgage Bonds Due 2033
Debt Instrument [Line Items]
Debt Instrument, Face Amount		$ 239,000
Interest rate, stated percentage		5.57%
Long-term Debt, Maturity Date		Mar. 30, 2033
South Dakota First Mortgage Bonds Due 2033
Debt Instrument [Line Items]
Debt Instrument, Face Amount	$ 31,000
Interest rate, stated percentage	5.57%
Long-term Debt, Maturity Date	Mar. 30, 2033
South Dakota First Mortgage Bonds Due 2033, Issued May 2023
Debt Instrument [Line Items]
Debt Instrument, Face Amount	$ 30,000
Secured Debt | Secured Debt Montana Due 2030
Debt Instrument [Line Items]
Interest rate, stated percentage	3.21%
Secured Debt | Secured Debt South Dakota Due 2030
Debt Instrument [Line Items]
Interest rate, stated percentage	3.21%
Secured Debt | Montana First Mortgage Bonds Due 2024
Debt Instrument [Line Items]
Interest rate, stated percentage	1.00%
Secured Debt | Secured Debt Montana 3.88% Due 2028
Debt Instrument [Line Items]
Interest rate, stated percentage	2.00%
Secured Debt | Montana 2.00% Due 2023
Debt Instrument [Line Items]
Interest rate, stated percentage	2.00%
Secured Debt | Montana First Mortgage Bonds Due 2033
Debt Instrument [Line Items]
Interest rate, stated percentage	5.57%
Secured Debt | South Dakota First Mortgage Bonds Due 2033
Debt Instrument [Line Items]
Interest rate, stated percentage	5.57%
Secured Debt | South Dakota First Mortgage Bonds Due 2033, Issued May 2023
Debt Instrument [Line Items]
Interest rate, stated percentage	5.42%