Enviromental matters (Details 2) (USD $)	12 Months Ended
	Dec. 31, 2014
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	12,500,000
Bellingham, WA manufactured gas plant site [Member]
Site Contingency [Line Items]
Site contingency, loss exposure not accrued, best estimate	$ 8,000,000