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LONG-TERM INCENTIVE AND RETIREMENT PLANS - Additional Information (Detail) - USD ($)
9 Months Ended 12 Months Ended 180 Months Ended
Apr. 15, 2019
Sep. 30, 2019
Sep. 30, 2018
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2018
Aug. 22, 2018
Aug. 21, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
Share-based compensation arrangement by share-based payment award, award vesting period 4 years              
Total compensation expense for unit awards   $ 2,814,000 $ 2,026,000 $ 2,523,000 $ 1,045,000      
Unit appreciation rights                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
UARs granted under LTIP, contractual term       5 years        
Share-based compensation arrangement by share-based payment award, award vesting period       48 months        
Common unit awards outstanding, intrinsic values       $ 0   $ 0    
Common unit awards outstanding, weighted average remaining contractual life       1 month 6 days        
Total compensation expense for unit awards       $ 100,000 100,000      
Unrecognized compensation cost related to non-vested UARs       0   $ 0    
Restricted Units                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
Total compensation expense for unit awards       400,000        
Restricted phantom units                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
Total compensation expense for unit awards       $ 2,000,000 $ 400,000      
2018 Plan                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
Long-Term Incentive Plan, common units permitted for grant (in shares)       2,000,000   2,000,000 2,000,000 1,500,000
Long-Term Incentive Plan, common units increase per year (in shares)       100,000        
Long-Term Incentive Plan, common units to be issued upon vesting and exercise (in shares)       1,122,601   1,122,601    
Long-Term Incentive Plan, common units, issued (in shares)       34,036        
Long-Term Incentive Plan, common units available for future issuance (in shares)       843,363   843,363    
Long-Term Incentive Plan, common units increased for issuance during the period (in shares)       0        
2018 Plan | Restricted phantom units                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
Common unit awards outstanding, intrinsic values       $ 2,400,000   $ 2,400,000    
2004 Plan                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
Long-Term Incentive Plan, common units to be issued upon vesting and exercise (in shares)       219,306   219,306    
Long-Term Incentive Plan, common units, issued (in shares)           626,188    
Long-Term Incentive Plan, common units available for future issuance (in shares)         0      
Long-Term Incentive Plan, expiration date       Sep. 10, 2014        
2004 Plan | Restricted phantom units                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
Common unit awards outstanding, intrinsic values       $ 500,000   $ 500,000