Mineral properties and development costs (Narrative) (Details) - USD ($) $ in Thousands	1 Months Ended			3 Months Ended		8 Months Ended	9 Months Ended		149 Months Ended
	Oct. 31, 2011	Jan. 31, 2011	Jan. 31, 2010	Aug. 31, 2016	Aug. 31, 2015	Aug. 05, 2012	Aug. 31, 2016	Aug. 31, 2015	Aug. 31, 2016	Jan. 31, 2012	Oct. 19, 2011	Aug. 05, 2011	Jan. 11, 2010	Dec. 31, 2009
Mineral properties expense				$ 3,077	$ 2,771		$ 4,067	$ 3,389
Approximations [Member]
Payments for mineral property									$ 92,600
Acquisition costs									30,600
Mineral properties expense									62,000
Ambler Property [Member]
Mineral property interest percentage													100.00%	51.00%
Mineral Property, Shares Issued, Shares			931,098
Mineral Property, Shares Issued, Value			$ 5,000
Payments for mineral property		$ 12,000
Fair value of cash payments to acquire mineral properties		$ 11,100								$ 10,300
Discount rate used		8.00%
Mineral property, fair value of consideration						$ 26,600
Transaction costs												$ 100
Net smelter return royalty													1.00%
Purchase price of the net smelter royalty													$ 10,000
Bornite Property [Member]
Payments for mineral property	$ 4,000
Net proceeds royalty											15.00%
Discount on consideration				$ 40,000			$ 40,000		$ 40,000
Bornite Property [Member] | Minimum [Member]
Mineral property interest percentage											16.00%
Net smelter return royalty											1.00%
Bornite Property [Member] | Maximum [Member]
Mineral property interest percentage											25.00%
Net smelter return royalty											2.50%