Environmental matters (Details 2) - USD ($)	9 Months Ended
	Sep. 30, 2016	Jun. 08, 2016
Portland Harbor Site [Member]
Site Contingency [Line Items]
Environmental matters investigative costs	$ 100,000,000
Environmental matters, estimated costs		$ 746,000,000
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	12,700,000
Bellingham, WA Manufactured Gas Plant Site [Member]
Site Contingency [Line Items]
Site contingency, loss exposure not accrued, best estimate	$ 8,000,000