Employee Benefit Plans - Amortization Schedule (Details) - USD ($) $ in Thousands	12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Retiree Medical Plan
Defined Benefit Plan, Accumulated Other Comprehensive Income (Loss), Net Prior Service Cost (Credit) [Roll Forward]
Net Prior Service Cost (Credit), beginning of period	$ (10,451)
Annual Amortization	1,758
Net Prior Service Cost (Credit), end of period	(8,693)	$ (10,451)
Amortization of Net (Gain) Loss Calculation
Net (gain) loss	(9,206)	(10,298)
Net (gain) loss subject to amortization	(9,206)	(10,298)
Corridor (10% of greater of APBO or assets)	1,280	1,214
Net (gain)/loss in excess of corridor	$ (7,926)	$ (9,084)
Amortization years	8 years 10 months 24 days	9 years 8 months 12 days
Other Postretirement Benefit Plans, Defined Benefit Plan Established January 1, 2008 [Member]
Defined Benefit Plan, Accumulated Other Comprehensive Income (Loss), Net Prior Service Cost (Credit) [Roll Forward]
Net Prior Service Cost (Credit), beginning of period	$ (502)
Annual Amortization	$ 231
Years Remaining	1 year 2 months 12 days
Curtailment	$ 0
Net Prior Service Cost (Credit), end of period	(271)	$ (502)
Other Postretirement Benefit Plans, Defined Benefit Plan Established July 1, 2012 [Member]
Defined Benefit Plan, Accumulated Other Comprehensive Income (Loss), Net Prior Service Cost (Credit) [Roll Forward]
Net Prior Service Cost (Credit), beginning of period	(9,949)
Annual Amortization	$ 1,527
Years Remaining	5 years 6 months
Curtailment	$ 0
Net Prior Service Cost (Credit), end of period	(8,422)	(9,949)
Death Benefit Plan
Amortization of Net (Gain) Loss Calculation
Net (gain) loss	1,201	1,523
Net (gain) loss subject to amortization	1,201	1,523
Corridor (10% of greater of APBO or assets)	(848)	(854)
Net (gain)/loss in excess of corridor	$ 353	$ 669
Amortization years	6 years 4 months 24 days	7 years