Employee Benefit Plans - Components of Net Periodic Benefit Cost and Amounts Recognized in Other Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2017	Dec. 31, 2016
Pension Plan
Components of net periodic benefit cost
Service cost	$ 0	$ 124	$ 0	$ 248
Interest cost	1,432	1,397	2,864	2,794
Expected return on plan assets	(1,456)	(1,607)	(2,912)	(3,214)
Amortization of net loss (gain)	418	508	836	1,016
Net periodic benefit (credit) cost	394	422	$ 788	$ 844
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	$ 304	$ 380
Interest cost	209	207	418	414
Amortization of net loss (gain)	(210)	(157)	(420)	(314)
Amortization of prior service credit	(439)	(439)	(878)	(878)
Net periodic benefit (credit) cost	$ (288)	$ (199)	$ (576)	$ (398)
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%