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, 2017	Sep. 30, 2016
Pension Plan
Components of net periodic benefit cost
Service cost	$ 0	$ 124
Interest cost	1,432	1,397
Expected return on plan assets	(1,456)	(1,607)
Amortization of net loss (gain)	418	508
Net periodic benefit (credit) cost	$ 394	$ 422
Weighted average assumptions used to determine benefit obligations
Discount rate	3.80%	3.55%
Expected long-term return on plan assets	6.75%	7.75%
Other Postretirement Benefit Plan
Components of net periodic benefit cost
Service cost	$ 152	$ 190
Interest cost	209	207
Amortization of net loss (gain)	(210)	(157)
Amortization of prior service credit	(439)	(439)
Net periodic benefit (credit) cost	$ (288)	$ (199)
Retiree Medical Plan
Weighted average assumptions used to determine benefit obligations
Discount rate	4.13%	3.73%
Death Benefit Plan
Weighted average assumptions used to determine benefit obligations
Discount rate	4.12%	3.79%