Retirement benefits - Net pension benefit expense (Details) - Pension Plans - USD ($) $ in Thousands	3 Months Ended
	Sep. 30, 2020	Sep. 30, 2019
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	$ 22,810	$ 19,549
Interest cost	25,417	33,993
Expected return on plan assets	(66,402)	(63,895)
Amortization of prior service cost	843	1,509
Amortization of net actuarial loss	52,331	39,561
Amortization of initial net obligation	5	4
Net pension benefit expense	$ 35,004	$ 30,721