Commitments and Contingencies - Environmental matters (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2023 USD ($)
Missoula, MT manufactured gas plant site
Site Contingency [Line Items]
Total estimated costs for site remediation	$ 560
Environmental matters estimated investigative costs	1,800
Environmental matters accrual for site remediation	610
Incurred costs for site remedial investigation	1,000
Bremerton, WA manufactured gas plant site
Site Contingency [Line Items]
Environmental matters accrual for site remediation	17,500
Total estimated costs for site remedial investigation and feasibility study	13,700
Incurred costs for site remedial investigation and feasibility study	10,600
Environmental matters accrual of investigative costs	3,200
Bellingham, WA manufactured gas plant site
Site Contingency [Line Items]
Site contingency, loss exposure not accrued, preferred alternative estimate	9,300
Minimum | Bremerton, WA manufactured gas plant site
Site Contingency [Line Items]
Site contingency, loss exposure not accrued, best estimate	13,600
Minimum | Bellingham, WA manufactured gas plant site
Site Contingency [Line Items]
Site contingency, loss exposure not accrued, best estimate	8,000
Maximum | Bremerton, WA manufactured gas plant site
Site Contingency [Line Items]
Site contingency, loss exposure not accrued, best estimate	71,500
Maximum | Bellingham, WA manufactured gas plant site
Site Contingency [Line Items]
Site contingency, loss exposure not accrued, best estimate	$ 20,400