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Leases - Additional Information (Details) | 12 Months Ended
--- | ---
Dec. 31, 2020
Lessee, Lease, Description [Line Items] |
Operating lease, existence of option to extend | true
Minimum [Member] |
Lessee, Lease, Description [Line Items] |
Operating lease, remaining terms of contract | 1 year
Operating lease, options to extend lease term | 5 years
Maximum [Member] |
Lessee, Lease, Description [Line Items] |
Operating lease, remaining terms of contract | 73 years
Operating lease, options to extend lease term | 50 years
Snettisham Hydroelectric Project [Member] |
Lessee, Lease, Description [Line Items] |
Finance lease expiration year | 2034
State of Montana [Member] | Hydroelectric Facilities in the Clark Fork River Basin [Member] |
Lessee, Lease, Description [Line Items] |
Operating lease expiration year | 2046
ASC 842 [Member] |
Lessee, Lease, Description [Line Items] |
Change in accounting principle, accounting standards update, adopted | true
Change in accounting principle, accounting standards update, adoption date | Jan. 01, 2019

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