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Revenue Recognition (Tables)
 3 Months Ended
Mar. 31, 2019
Revenue from Contract with Customer [Abstract]  
Disaggregation of Revenue
The following table disaggregates revenue by major sources:
 
THREE MONTHS ENDED
 
MARCH 31
Major Goods/Service Lines
2019
 
2018
Coal Mining
$
16,750

 
$
17,597

NAMining
10,775

 
10,213

Minerals Management
12,686

 
3,476

Unallocated Items
543

 
—

Eliminations
(657
)
 
(86
)
Total revenues
$
40,097

 
$
31,200

 
 
 
 
Timing of Revenue Recognition
 
 
 
Goods transferred at a point in time
$
16,086

 
$
17,021

Services transferred over time
24,011

 
14,179

Total revenues
$
40,097

 
$
31,200
Contract Balances
The opening and closing balances of the Company’s current and long-term contract liabilities and receivables are as follows:
 
Contract balances
 
Trade accounts receivable, net
 
Contract liability (current)
 
Contract liability (long-term)
Balance, January 1, 2019
$
20,817

 
$
754

 
$
2,008

Balance, March 31, 2019
19,341

 
718

 
1,832

Increase (decrease)
$
(1,476
)
 
$
(36
)
 
$
(176
)