STOCK-BASED COMPENSATION - NON-OPTIONS (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2018
Other stock-based compensation
Contractual term of awards	10 years
Minimum
Other stock-based compensation
Continuous service period for awards to vest	1 year
Maximum
Other stock-based compensation
Continuous service period for awards to vest	3 years
Stock appreciation rights
Non-vested other than stock options, number of shares
Outstanding at the beginning of the period (in shares)	175,599
Granted (in shares)	53,616
Exercised (in shares)	(101,823)
Forfeited (in shares)	(4,788)
Outstanding at the end of the period (in shares)	122,604	175,599
Exercisable at the end of the period (in shares)	42,386
Other than stock options, weighted-average grant date fair value
Non-vested at the beginning of the period (in dollars per share)	$ 79.20
Granted (in dollars per share)	124.15
Exercised (in dollars per share)	81.02
Forfeited (in dollars per share)	124.60
Non-vested at the end of the period (in dollars per share)	95.57	$ 79.20
Exercisable at the end of the period (in dollars per share)	$ 73.48
Other than stock options, weighted-average remaining contractual life (in years)
Outstanding at the end of the period	7 years 9 months 18 days
Exercisable at the end of the period	6 years 3 months 18 days
Aggregate Intrinsic Value
Intrinsic value on outstanding shares	$ 3,536
Intrinsic value on exercisable shares	$ 2,155
Other stock-based compensation
Fair value granted (in dollars per share)	$ 32.33	$ 26.37	$ 29.17
Total intrinsic value	$ 4,600	$ 2,800	$ 6,400
Unrecognized compensation expense	$ 1,500
Weighted-average vesting period (years)	1 year 10 months 24 days
Restricted stock
Non-vested other than stock options, number of shares
Outstanding at the beginning of the period (in shares)	142,969
Granted (in shares)	26,026
Vested (in shares)	(63,054)
Forfeited (in shares)	(9,894)
Outstanding at the end of the period (in shares)	96,047	142,969
Other than stock options, weighted-average grant date fair value
Non-vested at the beginning of the period (in dollars per share)	$ 77.13
Granted (in dollars per share)	123.72
Vested (in dollars per share)	92.90
Forfeited (in dollars per share)	121.12
Non-vested at the end of the period (in dollars per share)	$ 90.40	$ 77.13
Other stock-based compensation
Unrecognized compensation expense	$ 3,300
Weighted-average vesting period (years)	3 years 1 month 6 days
Restricted stock | Officers and Certain Employees
Non-vested other than stock options, number of shares
Granted (in shares)	17,710
Other stock-based compensation
Vesting period	3 years
Holding period of awards as a vesting condition	2 years
Vesting (as a percent)	33.00%
Restricted stock | Non Executive Directors
Non-vested other than stock options, number of shares
Granted (in shares)	8,316
Other stock-based compensation
Vesting period	1 year
Vesting (as a percent)	50.00%
Percentage of shares granted to non-executive directors, vesting one year after date of grant	50.00%
Performance stock
Non-vested other than stock options, number of shares
Outstanding at the beginning of the period (in shares)	155,022
Granted (in shares)	32,840
Vested (in shares)	(15,753)
Forfeited (in shares)	(15,525)
Non-attainment (in shares)	(11,672)
Outstanding at the end of the period (in shares)	144,912	155,022
Other than stock options, weighted-average grant date fair value
Non-vested at the beginning of the period (in dollars per share)	$ 68.35
Granted (in dollars per share)	100.48
Vested (in dollars per share)	53.65
Forfeited (in dollars per share)	98.15
Non-attainment (in dollars per share)	52.79
Non-vested at the end of the period (in dollars per share)	$ 80.59	$ 68.35
Other stock-based compensation
Unrecognized compensation expense	$ 1,500
Weighted-average vesting period (years)	2 years 2 months 12 days
Continuous service period for awards to vest	3 years
Earn out basis if no goals are met (as a percent)	0.00%
Earn out basis if some goals are met (as a percent)	100.00%
Earn out basis if all goals are met (as a percent)	200.00%
Performance stock | Vesting upon achievement of annual growth in Net GEOs
Other stock-based compensation
Vesting (as a percent)	50.00%
Performance stock | Vesting upon achievement of annual growth in Net TSR
Other stock-based compensation
Vesting (as a percent)	50.00%
Gold Equivalent Ounces Shares
Other stock-based compensation
Contractual term of awards	5 years
Total Shareholder Return Shares
Other stock-based compensation
Fair value granted (in dollars per share)	$ 77.50
Contractual term of awards	3 years
Continuous service period for awards to vest	3 years