LONG-TERM DEBT - Equipment Term Loans and Mortgages (Details)	1 Months Ended	3 Months Ended		6 Months Ended
	Apr. 30, 2016 USD ($) item	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) Lender	Jun. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)	Nov. 30, 2017 USD ($)	Feb. 28, 2017 USD ($)
LONG-TERM DEBT
Loan balance		$ 632,231,000		$ 632,231,000		$ 635,007,000
Deferred finance charges							$ 4,800,000	$ 14,200,000
Book value recorded under capital leases		5,600,000		5,600,000		7,000,000
Accumulated depreciation		7,100,000		7,100,000		5,800,000
Depreciation expense leased equipment		700,000	$ 1,500,000	1,300,000	$ 3,000,000
Equipment term loans
LONG-TERM DEBT
Equipment with collateralizes term loans		125,600,000		125,600,000
Loan balance		$ 129,786,000		$ 129,786,000		$ 126,227,000
Number of lenders | Lender				37
Equipment term loans | Minimum
LONG-TERM DEBT
Interest rate (as a percent)		1.40%		1.40%
Equipment term loans | Maximum
LONG-TERM DEBT
Interest rate (as a percent)		10.80%		10.80%
Bank mortgage loan
LONG-TERM DEBT
Interest rate (as a percent)		3.70%		3.70%
Loan amount		$ 3,600,000		$ 3,600,000
Monthly installments				15,776
Real estate term loan
LONG-TERM DEBT
Loan amount	$ 14,200,000
Monthly installments				$ 59,109
Number of real estate assets | item	9
Term loan amortization period				20 years
Deferred finance charges		$ 400,000		$ 400,000
Debt issuance cost, amortization period				5 years
Real estate term loan | LIBOR
LONG-TERM DEBT
Basis spread on variable rate	2.75%
November 1 2020 | Equipment term loans
LONG-TERM DEBT
Number of days of adjustment of interest rate and monthly payments				45 days
November 1 2020 | Equipment term loans | Base Rate
LONG-TERM DEBT
Interest rate (as a percent)		2.50%		2.50%
November 1 2020 | Equipment term loans | Minimum
LONG-TERM DEBT
Interest rate (as a percent)		3.70%		3.70%