Retirement benefits - Net pension benefit cost recognized (Details) - Pension Plans - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plans Disclosure
Service cost	$ 18,242	$ 23,116	$ 41,300	$ 47,339
Interest cost	36,138	29,972	71,824	61,468
Expected return on plan assets	(65,350)	(60,180)	(128,526)	(117,797)
Amortization of prior service cost	1,511	1,683	3,615	3,399
Amortization of net actuarial loss	35,468	54,325	74,150	105,752
Amortization of initial net obligation	5	5	10	10
Net pension benefit cost	$ 26,014	$ 48,921	$ 62,373	$ 100,171