Retirement benefits - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2019	Dec. 31, 2018
Other Postretirement Benefit Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Other postretirement benefits expense	$ 510	$ 631	$ 967	$ 1,281