LONG-TERM DEBT - Additional information (Details) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2018	Dec. 31, 2013	Dec. 31, 2017	Jun. 30, 2017
Long-term Debt and Capital Lease Obligations, Including Current Maturities [Abstract]
2018	$ 48,391
2019	38,336
2020	27,132
2021	17,979
2022	13,208
Thereafter	487,792
Loan amount attributable to interest	(607)
Total	567,378		$ 574,301
Long-term debt and capital Lease Obligations, Total	632,231
Equipment term loans
Long-term Debt and Capital Lease Obligations, Including Current Maturities [Abstract]
2018	41,092
2019	34,081
2020	23,716
2021	14,610
2022	10,207
Thereafter	6,080
Total	129,786
Capital leases
Long-term Debt and Capital Lease Obligations, Including Current Maturities [Abstract]
2018	2,314
2019	1,755
2020	916
2021	869
2022	501
Thereafter	727
Total minimum lease payments	7,082
Loan amount attributable to interest	(607)
Total (Present value of minimum lease payments on capital leases)	6,475
Less current portion	(1,945)
Long-term capital leases	$ 4,530
Senior subordinated secured debt | Main Street Capital Corporation
Loan amount		$ 20,000
Interest rate (as a percent)		12.00%
Paid-in-kind interest (as a percent)		2.50%
Accrued, Paid-in-kind interest added to principal balance				$ 100
Senior subordinated secured debt | Prudential Capital Partners
Loan amount		$ 20,000
Interest rate (as a percent)		12.00%
Paid-in-kind interest (as a percent)		2.50%
Accrued, Paid-in-kind interest added to principal balance				$ 100
Subordinated notes
Make-whole payment	5.00%
Subordinated notes | LST Seller notes
Loan amount	$ 22,000
Interest rate (as a percent)	10.00%
Subordinated notes | DTR Seller notes
Loan amount	$ 1,000
Interest rate (as a percent)	5.00%
Subordinated notes | BHE Seller notes
Loan amount	$ 2,000
Interest rate (as a percent)	7.00%
Term loan facility
Long-term Debt and Capital Lease Obligations, Including Current Maturities [Abstract]
2018	$ 4,985
2019	2,500
2020	2,500
2021	2,500
2022	2,500
Thereafter	480,985
Total	$ 495,970