Long-Term Debt - Other Secured Obligations (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017
Long Term Debt
Outstanding principal balance	$ 2,401,393	$ 2,408,852
Minnesota Economic Development State Loans
Long Term Debt
Outstanding principal balance	$ 18,900	20,600
Interest rate percent after February 2017	5.00%
Electric Utility Development Loans
Long Term Debt
Stated interest rate	8.10%
Outstanding principal balance	$ 2,800	3,600
Senior secured revolving credit facility
Long Term Debt
Unused stand-by letter of credit	1,200,000
Maximum borrowing capacity of credit facility	1,200,000
Standby letters of credit \| Electric Utility Development Loans
Long Term Debt
Unused stand-by letter of credit	$ 3,000
Revolving credit facility entered into by a controlled subsidiary
Long Term Debt
Interest rate on debt instrument	4.80%
Maximum borrowing capacity of credit facility	$ 40,000
Borrowings outstanding	$ 19,600	25,700
Secured Debt [Member]
Long Term Debt
Stated interest rate	6.00%
Outstanding principal balance	$ 8,300	$ 8,900

Represents the percentage of interest on the principal amount of notes after specified period.

No definition available.

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- Definition

Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 50
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- Definition

Effective interest rate for the funds borrowed under the debt agreement considering interest compounding and original issue discount or premium.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 50
-Paragraph 6
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=109500613&loc=SL6036836-161870

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- Definition

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

+ References

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- 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.

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- Definition

The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
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- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

+ References

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- Definition

Amount of borrowing capacity currently available under the credit facility (current borrowing capacity less the amount of borrowings outstanding).

+ References

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