Retirement benefits - Additional information (Details) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended		6 Months Ended
	Sep. 30, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2017
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		$ 631	$ 1,091	$ 1,281	$ 2,180