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Long-Term Debt and Capital Leases Schedule of Debt (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018
Debt Instrument [Line Items]
Long-term debt	$ 2,233,281			$ 2,102,345
Less current maturities	0			0
Long-term debt, excluding current maturities	2,233,281			2,102,345
Capital Lease
Total Capital Leases	19,915			22,213
Less current maturities	(2,476)			(2,298)
Capital lease obligations, noncurrent	17,439			19,915
Montana 3.98%, Due June 2049
Debt Instrument [Line Items]
Debt Instrument, Face Amount			$ 50,000
Montana 3.98%, Due September 2049
Debt Instrument [Line Items]
Debt Instrument, Face Amount		$ 100,000
Unsecured Debt \| Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Long-term debt	$ 289,000			290,000
Maturity date	Dec. 12, 2021
Unsecured Debt \| Swingline Credit Facility [Member]
Debt Instrument [Line Items]
Long-term debt	$ 0			18,000
Maturity date	Mar. 27, 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 \| 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
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
Maturity date	Sep. 17, 2049
Interest rate, stated percentage	3.98%
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	$ 26,977			26,977
Maturity date	Jul. 01, 2046
Interest rate, stated percentage	1.146%
Discount on Notes and Bonds and Debt Issuance Costs, Net
Debt Instrument [Line Items]
Discount on notes and bonds and debt issuance costs, net	$ (12,356)			$ (12,292)