Long-Term Debt and Capital Leases Schedule of Long-Term 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
Secured Debt \| Montana 4.03%, Due 2047
Debt Instrument [Line Items]
Long-term Debt	$ 250,000	250,000
Debt instrument, face amount		$ 250,000
Debt Instrument, issuance date		Nov. 06, 2017
Interest rate, stated percentage	4.03%	4.03%
Maturity date	Nov. 06, 2047	Nov. 06, 2047
Secured Debt \| Montana, 6.34%, Due 2019
Debt Instrument [Line Items]
Repurchased face amount, debt instrument		$ 250,000
Interest rate, stated percentage		6.34%
Maturity date		Apr. 01, 2019

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

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

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Date the debt instrument was issued, in CCYY-MM-DD format.

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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.

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

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Face (par) amount of the original debt instrument that was repurchased.

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

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