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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, 2016
Mar. 31, 2015
Mar. 31, 2016
Mar. 31, 2015
Pension Plan [Member]        
Components of net periodic benefit cost        
Service cost $ 97 $ 97 $ 291 $ 291
Interest cost 1,546 1,415 4,638 4,245
Expected return on plan assets (1,710) (1,823) (5,130) (5,469)
Amortization of net loss(1) 370 303 1,110 909
Net periodic benefit cost (credit) 303 (8) $ 909 $ (24)
Weighted average assumptions used to determine benefit obligations        
Discount rate     4.40% 4.15%
Expected long-term return on plan assets     7.50% 7.50%
Other Postretirement Benefit Plan, Defined Benefit [Member]        
Components of net periodic benefit cost        
Service cost 347 299 $ 1,041 $ 897
Interest cost 299 235 897 705
Amortization of net loss(1) (49) (125) (147) (375)
Amortization of prior service cost (credit) (439) (439) (1,317) (1,317)
Net periodic benefit cost (credit) $ 158 $ (30) $ 474 $ (90)
Retiree Medical Plan        
Weighted average assumptions used to determine benefit obligations        
Discount rate     4.69% 4.29%
Death Benefit Plan        
Weighted average assumptions used to determine benefit obligations        
Discount rate     4.74% 4.48%