Retirement benefits - Net pension benefit expense (Details) - Pension Plans - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	$ 21,449	$ 17,983	$ 40,998	$ 38,492
Interest cost	35,851	40,551	69,844	80,417
Expected return on plan assets	(66,883)	(62,701)	(130,778)	(125,578)
Amortization of prior service cost	1,309	1,677	2,818	3,325
Amortization of net actuarial loss	42,779	30,692	82,340	59,985
Amortization of initial net obligation	5	4	9	9
Net pension benefit expense	$ 34,510	$ 28,206	$ 65,231	$ 56,650