Retirement benefits - Additional information (Details) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended
	Sep. 30, 2018	Sep. 30, 2018	Sep. 30, 2017
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Discretionary cash contribution to defined benefit plan	$ 200,000
Other Postretirement Benefit Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net pension benefit cost		$ 650	$ 1,089