Leases Leases (Details) $ in Millions	6 Months Ended
	Jun. 30, 2019 USD ($)
Lessee, Lease, Description [Line Items]
Lessor, Operating Lease, Not Yet Commenced, Assumption and Judgment, Estimated Annual Operating Expense	$ 1.1
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