Retirement benefits - Net pension (benefit) expense (Details) - Pension Plans - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Mar. 31, 2024	Mar. 31, 2023
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	$ 12,646	$ 14,599	$ 38,593	$ 42,534
Interest cost	67,644	59,021	202,077	166,456
Expected return on plan assets	(88,381)	(80,137)	(264,862)	(228,695)
Amortization of prior service cost	319	227	955	679
Amortization of net actuarial loss (gain)	1,992	(657)	5,967	12,625
Net pension (benefit) expense	$ (5,780)	$ (6,947)	$ (17,270)	$ (6,401)