LONG-TERM DEBT - Equipment Term Loans and Mortgages (Details)	12 Months Ended
	Dec. 31, 2019 USD ($) Lender	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Nov. 30, 2017 USD ($)	Feb. 28, 2017 USD ($)
LONG-TERM DEBT
Loan balance	$ 704,100,000	$ 702,400,000
Deferred finance charges				$ 4,800,000	$ 14,200,000
Book value recorded under capital leases		16,600,000
Accumulated depreciation		7,800,000
Depreciation expense leased equipment		2,900,000	$ 2,600,000
Equipment term loans
LONG-TERM DEBT
Equipment with collateralizes term loans	185,200,000
Loan balance	$ 188,400,000	$ 190,700,000
Number of lenders | Lender	33
Equipment term loans | Minimum
LONG-TERM DEBT
Interest rate (as a percent)	1.50%
Equipment term loans | Maximum
LONG-TERM DEBT
Interest rate (as a percent)	10.70%
Bank mortgage loan
LONG-TERM DEBT
Loan balance	$ 3,200,000
Interest rate (as a percent)	3.70%
Monthly installments	$ 15,776
November 2017 And 2020 | Equipment term loans
LONG-TERM DEBT
Number of days of adjustment of interest rate and monthly payments	45 days
November 2017 And 2020 | Equipment term loans | Base Rate
LONG-TERM DEBT
Interest rate (as a percent)	2.50%
November 2017 And 2020 | Equipment term loans | Minimum
LONG-TERM DEBT
Interest rate (as a percent)	3.70%