Long-Term Debt and Capital Leases Schedule of Debt (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Long-term debt	$ 2,102,345	$ 1,793,416
Less current maturities	0	0
Long-term debt, excluding current maturities	2,102,345	1,793,416
Capital Lease
Total Capital Leases	22,213	24,346
Less current maturities	(2,298)	(2,133)
Capital lease obligations, noncurrent	19,915	22,213
Unsecured Debt \| Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Long-term debt	$ 290,000	0
Maturity date	Dec. 12, 2021
Unsecured Debt \| Swingline Credit Facility [Member]
Debt Instrument [Line Items]
Long-term debt	$ 18,000	0
Maturity date	Mar. 27, 2020
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, 6.34%, Due 2019
Debt Instrument [Line Items]
Maturity date		Apr. 01, 2019
Interest rate, stated percentage		6.34%
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	4.176%
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	Nov. 06, 2047
Interest rate, stated percentage	4.03%	4.03%
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	$ (12,292)	$ (13,221)

Amount equal to the present value (the principal) at the beginning of the lease term of minimum lease payments during the lease term (excluding that portion of the payments representing executory costs such as insurance, maintenance, and taxes to be paid by the lessor, together with any profit thereon) net of payments or other amounts applied to the principal through the balance sheet date.

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