Environmental matters (Details 2) - USD ($)	3 Months Ended
	Mar. 31, 2019	Jan. 06, 2017
Portland Harbor Site
Site Contingency [Line Items]
Environmental matters investigative costs	$ 100,000,000
Environmental matters, estimated costs		$ 1,000,000,000
Eugene, OR manufactured gas plant site
Site Contingency [Line Items]
Total estimated costs for site remediation	3,500,000
Incurred costs for site remediation	$ 400,000
Estimated proportional share of cleanup liability	50.00%
Environmental matters accrual for site remediation	$ 1,500,000
Bremerton, WA manufactured gas plant site
Site Contingency [Line Items]
Environmental matters accrual for site remediation	6,400,000
Total estimated costs for site remedial investigation and feasibility study	7,600,000
Incurred costs for site remedial investigation and feasibility study	3,300,000
Environmental matters accrual of investigative costs	4,300,000
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	340,000
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	6,400,000
Maximum | Bellingham, WA manufactured gas plant site
Site Contingency [Line Items]
Site contingency, loss exposure not accrued, best estimate	$ 20,400,000