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