Retirement benefits - Net pension (benefit) expense (Details) - Pension Plans - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2023	Dec. 31, 2022
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	$ 12,420	$ 13,682	$ 25,947	$ 27,935
Interest cost	67,552	61,084	134,433	107,435
Expected return on plan assets	(88,233)	(82,213)	(176,481)	(148,558)
Amortization of prior service cost	334	242	636	452
Amortization of net actuarial loss	2,793	6,839	3,975	13,282
Net pension (benefit) expense	$ (5,134)	$ (366)	$ (11,490)	$ 546