Retirement Benefits - Assumptions used to measure net periodic benefit cost for postretirement plans (Details) - Other Postretirement Benefit Plans, Defined Benefit	12 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2018
Defined Benefit Plan Disclosure [Line Items]
Discount rate	3.15%	3.92%	3.46%
Ultimate medical cost trend rate	4.50%	4.50%	4.50%
Medical cost trend rate decreases to ultimate in year	2028	2026	2025
Discount rate used to measure benefit obligation	2.14%	3.15%
Defined benefit plan, participant age range, Pre-65
Defined Benefit Plan Disclosure [Line Items]
Current medical cost trend rate	7.09%	7.47%	8.19%
Defined benefit plan, participant age range, Post-65
Defined Benefit Plan Disclosure [Line Items]
Current medical cost trend rate	7.43%	7.87%	9.79%