Income Taxes - Components of the Temporary Differences (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2016
Deferred Tax Assets: [Abstract]
Postretirement benefits	$ 18,862	$ 29,813	$ 33,815
Accrued liabilities	4,754	7,885	11,760
Net operating loss carryforward	32,552	38,981	38,196
Other	6,728	6,824	6,952
Total deferred tax assets	62,896	83,503	90,723
Deferred tax liabilities: [Abstract]
Unrealized gain on investments	0	0	(609)
Fixed assets	(16,156)	(17,096)	(5,370)
Other	(5,536)	(10,861)	(11,609)
Total deferred tax liabilities	(21,692)	(27,957)	(17,588)
Valuation allowance	(1,896)	(1,613)	(1,627)
Net deferred tax assets (liabilities)	$ 39,308	$ 53,933	$ 71,508