Environmental matters (Details 2) - USD ($)	3 Months Ended
	Mar. 31, 2017	Jan. 06, 2017
Portland Harbor Site [Member]
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 [Member]
Site Contingency [Line Items]
Estimated proportional share of cleanup liability	50.00%
Accrual for environmental loss contingencies	$ 1,600,000
Bremerton, WA Manufactured Gas Plant Site [Member]
Site Contingency [Line Items]
Accrual for environmental loss contingencies	12,400,000
Bellingham, WA Manufactured Gas Plant Site [Member]
Site Contingency [Line Items]
Site contingency, loss exposure not accrued, best estimate	8,000,000
Minimum [Member] | Eugene, OR Manufactured Gas Plant Site [Member]
Site Contingency [Line Items]
Site contingency, loss exposure not accrued, best estimate	500,000
Minimum [Member] | Bremerton, WA Manufactured Gas Plant Site [Member]
Site Contingency [Line Items]
Site contingency, loss exposure not accrued, best estimate	340,000
Maximum [Member] | Eugene, OR Manufactured Gas Plant Site [Member]
Site Contingency [Line Items]
Site contingency, loss exposure not accrued, best estimate	11,000,000
Maximum [Member] | Bremerton, WA Manufactured Gas Plant Site [Member]
Site Contingency [Line Items]
Site contingency, loss exposure not accrued, best estimate	$ 6,400,000