Employee Benefit Plans - Components of Net Periodic Benefit Cost and Amounts Recognized in Other Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Pension Plan
Components of net periodic benefit cost
Service cost	$ 0	$ 0
Interest cost	1,426	1,432
Expected return on plan assets	(1,485)	(1,456)
Amortization of net loss (gain)	370	418
Net periodic benefit (credit) cost	$ 311	$ 394
Weighted average assumptions used to determine benefit obligations
Discount rate	4.05%	3.80%
Expected long-term return on plan assets	6.75%	6.75%
Other Postretirement Benefit Plan
Components of net periodic benefit cost
Service cost	$ 133	$ 152
Interest cost	222	209
Amortization of net loss (gain)	(209)	(210)
Amortization of prior service credit	(439)	(439)
Net periodic benefit (credit) cost	$ (293)	$ (288)
Retiree Medical Plan
Weighted average assumptions used to determine benefit obligations
Discount rate	4.25%	4.13%
Death Benefit Plan
Weighted average assumptions used to determine benefit obligations
Discount rate	4.25%	4.12%