REVENUE - Disaggregation (Details) $ in Thousands	3 Months Ended								12 Months Ended
	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2019 USD ($) item	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)
Disaggregation of Revenue
Number of revenue sources | item									2
Revenue	$ 115,694	$ 109,778	$ 97,592	$ 99,992	$ 116,235	$ 115,983	$ 114,348	$ 112,476	$ 423,056	$ 459,042	$ 440,814
Stream interests
Disaggregation of Revenue
Revenue									305,824	324,516	314,011
Stream interests | Mount Milligan
Disaggregation of Revenue
Revenue									101,010
Stream interests | Pueblo Viejo
Disaggregation of Revenue
Revenue									82,844
Stream interests | Andacollo
Disaggregation of Revenue
Revenue									69,264
Stream interests | Wassa and Prestea
Disaggregation of Revenue
Revenue									22,098
Stream interests | Rainy River
Disaggregation of Revenue
Revenue									22,142
Stream interests | Other
Disaggregation of Revenue
Revenue									8,466
Stream interests | Gold
Disaggregation of Revenue
Revenue									249,496
Stream interests | Silver
Disaggregation of Revenue
Revenue									33,282
Stream interests | Copper
Disaggregation of Revenue
Revenue									23,046
Royalty interests
Disaggregation of Revenue
Revenue									117,232	$ 134,526	$ 126,803
Royalty interests | Other
Disaggregation of Revenue
Revenue									91,984
Royalty interests | Penasquito
Disaggregation of Revenue
Revenue									13,865
Royalty interests | Cortez
Disaggregation of Revenue
Revenue									11,383
Royalty interests | Gold
Disaggregation of Revenue
Revenue									78,570
Royalty interests | Silver
Disaggregation of Revenue
Revenue									5,497
Royalty interests | Copper
Disaggregation of Revenue
Revenue									13,808
Royalty interests | Other
Disaggregation of Revenue
Revenue									$ 19,357