Long-Term Debt and Capital Leases Schedule of Debt (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2021	Dec. 31, 2020
Debt Instrument [Line Items]
Long-term Debt	$ 2,541,478	$ 2,315,261
Finance Lease, Liability, Total	14,772	17,439
Current maturities of finance leases	(2,875)	(2,668)
Long-term finance leases	11,897	14,771
Secured Debt South Dakota Due 2030
Debt Instrument [Line Items]
Debt Instrument, Face Amount	$ 50,000
Long-term Debt, Maturity Date	May 15, 2030
Secured Debt Montana Due 2030
Debt Instrument [Line Items]
Debt Instrument, Face Amount	$ 100,000
Long-term Debt, Maturity Date	May 15, 2030
Montana First Mortgage Bonds Due 2024
Debt Instrument [Line Items]
Debt Instrument, Face Amount	$ 100,000
Interest rate, stated percentage	1.00%
Long-term Debt, Maturity Date	Mar. 26, 2024
Unsecured Debt \| Revolving Credit Facility Due 2023
Debt Instrument [Line Items]
Long-term Debt	$ 373,000	222,000
Maturity date	Sep. 02, 2023
Unsecured Debt \| Revolving Credit Facility Due 2021
Debt Instrument [Line Items]
Maturity date	Dec. 12, 2021
Secured Debt \| South Dakota, 5.01%, Due 2025
Debt Instrument [Line Items]
Long-term Debt	$ 64,000	64,000
Maturity date	May 01, 2025
Interest rate, stated percentage	5.01%
Secured Debt \| South Dakota, 4.15%, Due 2042
Debt Instrument [Line Items]
Long-term Debt	$ 30,000	30,000
Maturity date	Aug. 10, 2042
Interest rate, stated percentage	4.15%
Secured Debt \| South Dakota, 4.30%, Due 2052
Debt Instrument [Line Items]
Long-term Debt	$ 20,000	20,000
Maturity date	Aug. 10, 2052
Interest rate, stated percentage	4.30%
Secured Debt \| South Dakota, 4.85% Due 2043
Debt Instrument [Line Items]
Long-term Debt	$ 50,000	50,000
Maturity date	Dec. 19, 2043
Interest rate, stated percentage	4.85%
Secured Debt \| South Dakota, 4.22% Due 2044
Debt Instrument [Line Items]
Long-term Debt	$ 30,000	30,000
Maturity date	Dec. 19, 2044
Interest rate, stated percentage	4.22%
Secured Debt \| South Dakota, 4.26% Due 2040
Debt Instrument [Line Items]
Long-term Debt	$ 70,000	70,000
Maturity date	Sep. 29, 2040
Interest rate, stated percentage	4.26%
Secured Debt \| Secured Debt South Dakota Due 2030
Debt Instrument [Line Items]
Long-term Debt	$ 50,000	50,000
Maturity date	May 15, 2030
Interest rate, stated percentage	3.21%
Secured Debt \| South Dakota, 2.80%, Due 2026
Debt Instrument [Line Items]
Long-term Debt	$ 60,000	60,000
Maturity date	Jun. 15, 2026
Interest rate, stated percentage	2.80%
Secured Debt \| South Dakota, 2.66%, Due 2026
Debt Instrument [Line Items]
Long-term Debt	$ 45,000	45,000
Maturity date	Sep. 30, 2026
Interest rate, stated percentage	2.66%
Secured Debt \| Montana, 5.71%, Due 2039
Debt Instrument [Line Items]
Long-term Debt	$ 55,000	55,000
Maturity date	Oct. 15, 2039
Interest rate, stated percentage	5.71%
Secured Debt \| Montana, 5.01%, Due 2025
Debt Instrument [Line Items]
Long-term Debt	$ 161,000	161,000
Maturity date	May 01, 2025
Interest rate, stated percentage	5.01%
Secured Debt \| Montana, 4.15%, Due 2042
Debt Instrument [Line Items]
Long-term Debt	$ 60,000	60,000
Maturity date	Aug. 10, 2042
Interest rate, stated percentage	4.15%
Secured Debt \| Montana, 4.30%, Due 2052
Debt Instrument [Line Items]
Long-term Debt	$ 40,000	40,000
Maturity date	Aug. 10, 2052
Interest rate, stated percentage	4.30%
Secured Debt \| Montana 4.85%, Due 2043
Debt Instrument [Line Items]
Long-term Debt	$ 15,000	15,000
Maturity date	Dec. 19, 2043
Interest rate, stated percentage	4.85%
Secured Debt \| Montana 3.99% Due 2028
Debt Instrument [Line Items]
Long-term Debt	$ 35,000	35,000
Maturity date	Dec. 19, 2028
Interest rate, stated percentage	3.99%
Secured Debt \| Montana 4.176% Due 2044
Debt Instrument [Line Items]
Long-term Debt	$ 450,000	450,000
Maturity date	Nov. 15, 2044
Interest rate, stated percentage	4.18%
Secured Debt \| Montana 3.11%, Due 2025
Debt Instrument [Line Items]
Long-term Debt	$ 75,000	75,000
Maturity date	Jul. 01, 2025
Interest rate, stated percentage	3.11%
Secured Debt \| Montana 4.11%, Due 2045
Debt Instrument [Line Items]
Long-term Debt	$ 125,000	125,000
Maturity date	Jul. 01, 2045
Interest rate, stated percentage	4.11%
Secured Debt \| Montana 4.03%, Due 2047
Debt Instrument [Line Items]
Long-term Debt	$ 250,000	250,000
Maturity date	Nov. 06, 2047
Interest rate, stated percentage	4.03%
Secured Debt \| Montana 3.98%, Due June 2049
Debt Instrument [Line Items]
Long-term Debt	$ 50,000	50,000
Maturity date	Jun. 26, 2049
Interest rate, stated percentage	3.98%
Secured Debt \| Montana 3.98%, Due September 2049
Debt Instrument [Line Items]
Long-term Debt	$ 100,000	100,000
Maturity date	Sep. 17, 2049
Interest rate, stated percentage	3.98%
Secured Debt \| Secured Debt Montana Due 2030
Debt Instrument [Line Items]
Long-term Debt	$ 100,000	100,000
Maturity date	May 15, 2030
Interest rate, stated percentage	3.21%
Secured Debt \| Montana 2.00% Due 2023
Debt Instrument [Line Items]
Long-term Debt	$ 144,660	144,660
Maturity date	Aug. 01, 2023
Interest rate, stated percentage	2.00%
Secured Debt \| New Market Tax Credit Financing-1.146%, Due 2046
Debt Instrument [Line Items]
Long-term Debt	$ 0	26,977
Maturity date	Jul. 01, 2046
Interest rate, stated percentage	1.146%
Secured Debt \| Secured Debt Montana Due 2049 [Member]
Debt Instrument [Line Items]
Long-term Debt	$ 150,000	150,000
Secured Debt \| Montana First Mortgage Bonds Due 2024
Debt Instrument [Line Items]
Long-term Debt	$ 100,000	0
Maturity date	Mar. 26, 2024
Interest rate, stated percentage	1.00%
Discount on Notes and Bonds and Debt Issuance Costs, Net
Debt Instrument [Line Items]
Discount on notes and bonds and debt issuance costs, net	$ (11,182)	$ (13,376)

Face (par) amount of debt instrument at time of issuance.

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