Mineral properties and development costs (Narrative) (Details) - USD ($) $ in Thousands				1 Months Ended	3 Months Ended			6 Months Ended		12 Months Ended		185 Months Ended
	Apr. 10, 2017	Oct. 19, 2011	Jan. 11, 2010	Oct. 31, 2017	May 31, 2019	Feb. 28, 2019	May 31, 2018	May 31, 2019	May 31, 2018	Nov. 30, 2018	Nov. 30, 2017	May 31, 2019
Payments for mineral property						$ 31,000
Mineral properties expense					$ 2,906		$ 2,475	$ 4,441	$ 3,606
Maximum Subscription Price	$ 150,000
Cost, Maintenance	$ 10,000
Proceeds from Contributions from Affiliates								$ 10,200	$ 9,635
Subscription of Options Description	To subscribe for 50% of the JV, South32 must contribute a minimum of $150 million, plus (i) any amounts Trilogy spends on matched parallel funding to a maximum of $16 million over the three year period and (ii) $5 million if the option is exercised between April 1, 2018 and March 31, 2019 or $10 million if the option is exercised between April 1, 2019 and the expiration date of the option, less the amount of the initial funding contributed by South32 (the “Subscription Price”).
Approximations
Payments for mineral property												$ 129,600
Acquisition costs												30,600
Mineral properties expense												$ 99,000
Titiribi Property [Member]
Proceeds from Contributions from Affiliates				$ 400		10,200				$ 10,400	$ 10,000
Proceeds From Advances from Affiliates										$ 800
Regional Exploration Program Budget Funding						$ 1,000
Bornite Property [Member]
Payments for mineral property		$ 4,000
Net proceeds royalty		15.00%
Discount on consideration		$ 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%
Ambler Property [Member]
Mineral property interest percentage			100.00%
Payments for mineral property			$ 10,000
Mineral property, fair value of consideration			$ 26,600
Net smelter return royalty			1.00%