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EMPLOYEE BENEFIT PLANS - Benefit Obligations, Plan Assets and Reported Amounts (Details) - ARLP - Pension Plan - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Change in benefit obligations:      
Benefit obligations at beginning of year $ 113,482 $ 107,476  
Service cost   2,205 $ 2,473
Interest cost 4,587 4,493 4,296
Actuarial (gain) loss 13,501 901  
Benefits paid (4,272) (3,091)  
Plan amendments   1,498  
Benefit obligations at end of year 127,298 113,482 107,476
Change in plan assets:      
Fair value of plan assets at beginning of year 71,412 68,445  
Employer contribution 2,971 2,608  
Actual return on plan assets 11,870 3,450  
Defined Benefit Plan, Plan Assets, Benefits Paid (4,272) (3,091)  
Fair value of plan assets at end of year 81,981 71,412 $ 68,445
Funded status at the end of year (45,317) (42,070)  
Amounts recognized in balance sheet:      
Non-current liability (45,317) (42,070)  
Amounts recognized in accumulated other comprehensive income consist of:      
Prior service credit (1,312) (1,498)  
Net actuarial loss (gain) (41,979) (38,424)  
Amounts recognized in accumulated other comprehensive income $ (43,291) $ (39,922)  
Weighted-average assumptions to determine benefit obligations      
Discount rate 3.54% 4.06%  
Expected rate of return on plan assets 7.00% 7.00%  
Weighted-average assumptions used to determine net periodic benefit cost      
Discount rate 4.06% 4.27%  
Expected return on plan assets 7.00% 7.50%