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v3.19.1

Leases Leases (Details) | 3 Months Ended
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Mar. 31, 2019
Lessee, Lease, Description [Line Items] |
Operating Lease, Right-of-Use Asset, Statement of Financial Position [Extensible List] | us-gaap:InvestmentsAndOtherNoncurrentAssets
Finance Lease, Right-of-Use Asset, Statement of Financial Position [Extensible List] | us-gaap:InvestmentsAndOtherNoncurrentAssets
Operating Lease, Liability, Current, Statement of Financial Position [Extensible List] | us-gaap:OtherLiabilitiesCurrent
Operating Lease, Liability, Noncurrent, Statement of Financial Position [Extensible List] | us-gaap:OtherLiabilitiesNoncurrent
Finance Lease, Liability, Current, Statement of Financial Position [Extensible List] | us-gaap:OtherLiabilitiesCurrent
Finance Lease, Liability, Noncurrent, Statement of Financial Position [Extensible List] | us-gaap:OtherLiabilitiesNoncurrent
Minimum [Member] |
Lessee, Lease, Description [Line Items] |
Lessee, Operating Lease, Term of Contract | 1 year
Lessee, Operating Lease, Renewal Term | 5 years
Maximum [Member] |
Lessee, Lease, Description [Line Items] |
Lessee, Operating Lease, Term of Contract | 74 years
Lessee, Operating Lease, Renewal Term | 50 years
State of Montana [Member] |
Lessee, Lease, Description [Line Items] |
Lease Expiration Date | Dec. 31, 2046
Snettisham Hydroelectric Project [Member] |
Lessee, Lease, Description [Line Items] |
Lease Expiration Date | Dec. 31, 2034

X
- Definition Indicates line item in statement of financial position that includes current finance lease liability. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 842 -SubTopic 20 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=77888419&loc=SL77918631-209977 + Details
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Namespace Prefix: | us-gaap_
Data Type: | srt-types:extensibleListItemType
Balance Type: | na
Period Type: | instant

X
- Definition Indicates line item in statement of financial position that includes noncurrent finance lease liability. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 842 -SubTopic 20 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=77888419&loc=SL77918631-209977 + Details
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Data Type: | srt-types:extensibleListItemType
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X
- Definition Indicates line item in statement of financial position that includes finance lease right-of-use asset. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 842 -SubTopic 20 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=77888419&loc=SL77918631-209977 + Details
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Balance Type: | na
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X
- Definition Date which lease or group of leases is set to expire, in CCYY-MM-DD format. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:dateItemType
Balance Type: | na
Period Type: | duration

X
- 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_LesseeLeaseDescriptionLineItems
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Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Term of lessee's operating lease renewal, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 842 -SubTopic 20 -Section 50 -Paragraph 3 -Subparagraph (a)(3) -URI http://asc.fasb.org/extlink&oid=77888426&loc=SL77918673-209980 + Details
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- Definition Term of lessee's operating lease, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 842 -SubTopic 20 -Section 50 -Paragraph 3 -Subparagraph (a)(3) -URI http://asc.fasb.org/extlink&oid=77888426&loc=SL77918673-209980 + Details
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Period Type: | instant

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- Definition Indicates line item in statement of financial position that includes current operating lease liability. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 842 -SubTopic 20 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=77888419&loc=SL77918631-209977 + Details
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- Definition Indicates line item in statement of financial position that includes noncurrent operating lease liability. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 842 -SubTopic 20 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=77888419&loc=SL77918631-209977 + Details
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- Definition Indicates line item in statement of financial position that includes operating lease right-of-use asset. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 842 -SubTopic 20 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=77888419&loc=SL77918631-209977 + Details
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- Details
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- Details
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Namespace Prefix: |
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- Details
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