Enviromental matters (Details 2) - USD ($)	3 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2020
Portland Harbor Site
Site Contingency [Line Items]
Environmental matters investigative costs		$ 100,000,000
Environmental matters, estimated costs		1,000,000,000
Missoula, MT manufactured gas plant site
Site Contingency [Line Items]
Total estimated costs for site remediation		560,000
Environmental Matters Estimated Investigative Costs		800,000
Environmental matters accrual for site remediation		800,000
Incurred Costs For Site Remedial Investigation		130,000
Bremerton, WA manufactured gas plant site
Site Contingency [Line Items]
Environmental matters accrual for site remediation		17,500,000
Total estimated costs for site remedial investigation and feasibility study		7,600,000
Incurred costs for site remedial investigation and feasibility study		5,000,000.0
Environmental Matters Accrual For Site Remediation, Period Increase	$ 11,100,000
Environmental matters accrual of investigative costs		2,600,000
Bellingham, WA manufactured gas plant site
Site Contingency [Line Items]
Site contingency, loss exposure not accrued, preferred alternative estimate		9,300,000
Minimum | Bremerton, WA manufactured gas plant site
Site Contingency [Line Items]
Site contingency, loss exposure not accrued, best estimate		13,600,000
Minimum | Bellingham, WA manufactured gas plant site
Site Contingency [Line Items]
Site contingency, loss exposure not accrued, best estimate		8,000,000.0
Maximum | Bremerton, WA manufactured gas plant site
Site Contingency [Line Items]
Site contingency, loss exposure not accrued, best estimate		71,000,000.0
Maximum | Bellingham, WA manufactured gas plant site
Site Contingency [Line Items]
Site contingency, loss exposure not accrued, best estimate		$ 20,400,000