Retirement benefits - Net pension benefit expense (Details) - Pension Plans - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2020	Mar. 31, 2019
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	$ 20,967	$ 19,126	$ 61,965	$ 57,618
Interest cost	36,389	40,124	106,233	120,541
Expected return on plan assets	(68,058)	(62,844)	(198,836)	(188,422)
Amortization of prior service cost	1,411	1,665	4,229	4,990
Amortization of net actuarial loss	41,200	29,988	123,540	89,973
Amortization of initial net obligation	4	4	13	13
Net pension benefit expense	$ 31,913	$ 28,063	$ 97,144	$ 84,713