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

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 50
-Paragraph 1B
-Subparagraph (a)
-URI https://asc.fasb.org/extlink&oid=123466505&loc=SL123495323-112611

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI https://asc.fasb.org/extlink&oid=124435984&loc=d3e28551-108399

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI https://asc.fasb.org/extlink&oid=114775985&loc=d3e28878-108400

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 50
-Paragraph 1
-URI https://asc.fasb.org/extlink&oid=124429444&loc=SL124452920-239629

Reference 5: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 55
-Paragraph 69C
-URI https://asc.fasb.org/extlink&oid=123466577&loc=SL123495737-112612

Reference 6: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 55
-Paragraph 69B
-URI https://asc.fasb.org/extlink&oid=123466577&loc=SL123495735-112612

+ Details

Name:	us-gaap_DebtInstrumentFaceAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ References

+ Details

Name:	us-gaap_DebtInstrumentInterestRateStatedPercentage
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Date when the debt instrument is scheduled to be fully repaid, in YYYY-MM-DD format.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(2))
-URI https://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 50
-Paragraph 1B
-Subparagraph (d)
-URI https://asc.fasb.org/extlink&oid=123466505&loc=SL123495323-112611

Reference 3: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (bbb)(2)
-URI https://asc.fasb.org/extlink&oid=126976982&loc=d3e19207-110258

+ Details

Name:	us-gaap_DebtInstrumentMaturityDate
Namespace Prefix:	us-gaap_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of unamortized debt discount (premium) and debt issuance costs.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 55
-Paragraph 69C
-URI https://asc.fasb.org/extlink&oid=123466577&loc=SL123495737-112612

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 50
-Paragraph 4
-Subparagraph (b)(2)
-URI https://asc.fasb.org/extlink&oid=123466204&loc=SL6031897-161870

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 50
-Paragraph 1D
-Subparagraph (a)
-URI https://asc.fasb.org/extlink&oid=123466505&loc=SL123495340-112611

Reference 4: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 55
-Paragraph 69B
-URI https://asc.fasb.org/extlink&oid=123466577&loc=SL123495735-112612

Reference 5: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 3
-URI https://asc.fasb.org/extlink&oid=124435984&loc=d3e28555-108399

+ Details

Name:	us-gaap_DebtInstrumentUnamortizedDiscountPremiumAndDebtIssuanceCostsNet
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Present value of lessee's discounted obligation for lease payments from finance lease.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 842
-SubTopic 20
-Section 50
-Paragraph 6
-URI https://asc.fasb.org/extlink&oid=128292326&loc=SL77918701-209980

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 842
-SubTopic 20
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI https://asc.fasb.org/extlink&oid=123391704&loc=SL77918627-209977

+ Details

Name:	us-gaap_FinanceLeaseLiability
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Present value of lessee's discounted obligation for lease payments from finance lease, classified as current.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 842
-SubTopic 20
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI https://asc.fasb.org/extlink&oid=123391704&loc=SL77918627-209977

+ Details

Name:	us-gaap_FinanceLeaseLiabilityCurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Present value of lessee's discounted obligation for lease payments from finance lease, classified as noncurrent.

+ References

+ Details

Name:	us-gaap_FinanceLeaseLiabilityNoncurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount, excluding unamortized premium (discount) and debt issuance cost, of long-term debt. Excludes lease obligation.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 50
-Paragraph 4
-Subparagraph (b)(3)
-URI https://asc.fasb.org/extlink&oid=123466204&loc=SL6031897-161870

Reference 2: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 55
-Paragraph 69C
-URI https://asc.fasb.org/extlink&oid=123466577&loc=SL123495737-112612

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(16))
-URI https://asc.fasb.org/extlink&oid=126734703&loc=d3e572229-122910

Reference 4: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 55
-Paragraph 69B
-URI https://asc.fasb.org/extlink&oid=123466577&loc=SL123495735-112612

Reference 5: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(16))
-URI https://asc.fasb.org/extlink&oid=126897435&loc=d3e534808-122878

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 50
-Paragraph 1D
-Subparagraph (b)
-URI https://asc.fasb.org/extlink&oid=123466505&loc=SL123495340-112611

Reference 7: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22))
-URI https://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

+ Details

Name:	us-gaap_LongTermDebt
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt, classified as current. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.19,20)
-URI https://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

+ Details

Name:	us-gaap_LongTermDebtCurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Maturity date of long-term debt, in YYYY-MM-DD format.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (bbb)(2)
-URI https://asc.fasb.org/extlink&oid=126976982&loc=d3e19207-110258

+ Details

Name:	us-gaap_LongTermDebtMaturityDate
Namespace Prefix:	us-gaap_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	instant

Long-Term Debt and Capital Leases Schedule of Debt (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Mar. 31, 2021
Debt Instrument [Line Items]
Long-term Debt	$ 2,618,882	$ 2,541,478
Long-term Debt, Current Maturities	(144,525)	0
Finance Lease, Liability, Total	11,897	14,772
Current maturities of finance leases	(3,098)	(2,875)
Long-term finance leases	8,799	11,897
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	$ 0	373,000
Maturity date	Sep. 02, 2023
Unsecured Debt \| Revolving Credit Facility Due 2021
Debt Instrument [Line Items]
Maturity date	Dec. 12, 2021
Unsecured Debt \| Revolving Credit Facility Due 2024
Debt Instrument [Line Items]
Long-term Debt	$ 25,000	0
Unsecured Debt \| Revolving Credit Facility Due 2027
Debt Instrument [Line Items]
Long-term Debt	425,000	0
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]
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	100,000
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	$ (10,778)	$ (11,182)