Environmental matters (Details 2) - USD ($)		9 Months Ended
	Oct. 12, 2015	Sep. 30, 2015
Portland Harbor Site [Member]
Site Contingency [Line Items]
Environmental matters investigative costs		$ 70,000,000
Eugene, OR Manufactured Gas Plant Site [Member]
Site Contingency [Line Items]
Site contingency, low estimate, loss exposure		500,000
Site contingency, high estimate, loss exposure		$ 11,000,000
Estimated proportional share of cleanup liability		50.00%
Accrual for environmental loss contingencies		$ 1,700,000
Bremerton, WA Manufactured Gas Plant Site [Member]
Site Contingency [Line Items]
Site contingency, low estimate, loss exposure		340,000
Site contingency, high estimate, loss exposure		6,400,000
Accrual for environmental loss contingencies		12,200,000
Bellingham, WA Manufactured Gas Plant Site [Member]
Site Contingency [Line Items]
Site contingency, loss exposure not accrued, best estimate		$ 8,000,000
Subsequent event [Member] | Coos County Oregon [Member]
Site Contingency [Line Items]
Site contingency, loss exposure not accrued, best estimate	$ 160,000