Retirement benefits - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2017	Dec. 31, 2016
Other Postretirement Benefit Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net pension benefit cost	$ 1,091	$ 1,116	$ 2,180	$ 2,232