Retirement benefits - Additional information (Details) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2019	Mar. 31, 2018
Defined Benefit Plans and Other Postretirement Benefit Plans
Discretionary cash contribution to defined benefit plan	$ 200,000
Other Postretirement Benefit Plans
Defined Benefit Plans and Other Postretirement Benefit Plans
Net pension benefit cost		$ 114	$ (100)	$ 1,395	$ 2,080