Retirement benefits - Net pension (benefit) expense (Details) - Pension Plans - USD ($) $ in Thousands	3 Months Ended
	Sep. 30, 2023	Sep. 30, 2022
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	$ 13,527	$ 14,253
Interest cost	66,881	46,351
Expected return on plan assets	(88,248)	(66,345)
Amortization of prior service cost	302	210
Amortization of net actuarial loss	1,182	6,443
Amortization of initial net obligation	0	0
Net pension (benefit) expense	$ (6,356)	$ 912