Retirement benefits - Net pension benefit expense (Details) - Pension Plans - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2020	Dec. 31, 2019
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	$ 19,389	$ 21,449	$ 42,199	$ 40,998
Interest cost	25,755	35,851	51,172	69,844
Expected return on plan assets	(66,807)	(66,883)	(133,209)	(130,778)
Amortization of prior service cost	1,446	1,309	2,289	2,818
Amortization of net actuarial loss	52,829	42,779	105,160	82,340
Amortization of initial net obligation	4	5	9	9
Net pension benefit expense	$ 32,616	$ 34,510	$ 67,620	$ 65,231