Employee Benefit Plans - Components of Net Periodic Benefit Cost and Amounts Recognized in Other Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2018	Mar. 31, 2017
Pension Plan
Components of net periodic benefit cost
Service cost	$ 0	$ 124	$ 0	$ 372
Interest cost	1,432	1,397	4,296	4,191
Expected return on plan assets	(1,456)	(1,607)	(4,368)	(4,821)
Amortization of net loss (gain)	418	508	1,254	1,524
Net periodic benefit (credit) cost	394	422	$ 1,182	$ 1,266
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	$ 456	$ 570
Interest cost	209	207	627	621
Amortization of net loss (gain)	(210)	(157)	(630)	(471)
Amortization of prior service credit	(439)	(439)	(1,317)	(1,317)
Net periodic benefit (credit) cost	$ (288)	$ (199)	$ (864)	$ (597)
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%