DEBT - Components of Long-Term Debt and Capital Leases (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018
Debt Instrument [Line Items]
Finance leases maturing through December 2107; bearing interest at a weighted average of 5.2%	$ 18,364
Principal amount of debt	522,728	$ 555,249
Less—unamortized discount and debt issuance costs	9,406	10,950
Debt less unamortized discount and debt issuance costs	513,322	544,299
Less—current maturities of debt	4,301	2,298
Debt, less current portion	$ 509,021	542,001
Weighted-average discount rate - finance leases	5.00%
Revolving Credit Facility due May 2023 ("Revolving Credit Facility"); bearing interest at LIBOR plus 1.75%
Debt Instrument [Line Items]
Principle amount of debt	$ 26,900
Principal amount of debt		69,600
New York State Environmental Facilities Corporation Solid Waste Disposal Revenue Bonds Series 2014 ("New York Bonds 2014") due December 2044 - fixed rate interest period through 2029; bearing interest at 2.875%
Debt Instrument [Line Items]
Principle amount of debt	25,000
Principal amount of debt		25,000
Less—unamortized discount and debt issuance costs	$ 1,057	847
Stated interest rate	2.875%
New York State Environmental Facilities Corporation Solid Waste Disposal Revenue Bonds Series 2014R-2 ("New York Bonds 2014R-2") due December 2044 - fixed rate interest period through 2026; bearing interest at 3.125%
Debt Instrument [Line Items]
Principle amount of debt	$ 15,000
Principal amount of debt		15,000
Less—unamortized discount and debt issuance costs	$ 390	450
Stated interest rate	3.125%
Finance Authority of Maine Solid Waste Disposal Revenue Bonds Series 2005R-3 ("FAME Bonds 2005R-3") due January 2025 - fixed rate interest period through 2025; bearing interest at 5.25%
Debt Instrument [Line Items]
Principle amount of debt	$ 25,000
Principal amount of debt		25,000
Less—unamortized discount and debt issuance costs	$ 432	517
Stated interest rate	5.25%
Finance Authority of Maine Solid Waste Disposal Revenue Bonds Series 2015R-1 ("FAME Bonds 2015R-1") due August 2035 - fixed rate interest period through 2025; bearing interest at 5.125%
Debt Instrument [Line Items]
Principle amount of debt	$ 15,000
Principal amount of debt		15,000
Less—unamortized discount and debt issuance costs	$ 552	622
Stated interest rate	5.125%
Finance Authority of Maine Solid Waste Disposal Revenue Bonds Series 2015R-2 ("FAME Bonds 2015R-2") due August 2035 - fixed rate interest period through 2025; bearing interest at 4.375%
Debt Instrument [Line Items]
Principle amount of debt	$ 15,000
Principal amount of debt		15,000
Less—unamortized discount and debt issuance costs	$ 417	493
Stated interest rate	4.375%
Vermont Economic Development Authority Solid Waste Disposal Long-Term Revenue Bonds Series 2013 ("Vermont Bonds") due April 2036 - fixed rate interest period through 2028; bearing interest at 4.625%
Debt Instrument [Line Items]
Principle amount of debt	$ 16,000
Principal amount of debt		16,000
Less—unamortized discount and debt issuance costs	$ 541	595
Stated interest rate	4.625%
Business Finance Authority of the State of New Hampshire Solid Waste Disposal Revenue Bonds Series 2013 ("New Hampshire Bonds") due April 2029 - fixed rate interest period through 2029; bearing interest at 2.95%
Debt Instrument [Line Items]
Principle amount of debt	$ 11,000
Principal amount of debt		11,000
Less—unamortized discount and debt issuance costs	$ 539	308
Stated interest rate	2.95%
Finance leases maturing through December 2107; bearing interest at a weighted average of 5.0%
Debt Instrument [Line Items]
Principal amount of debt		11,248
Notes payable maturing through June 2027; bearing interest at a weighted average of 3.5%
Debt Instrument [Line Items]
Principle amount of debt	$ 5,464
Principal amount of debt		2,401
Weighted average interest rate	3.50%
Credit Facility
Debt Instrument [Line Items]
Less—unamortized discount and debt issuance costs	$ 5,478	7,118
Term Loan A Facility due May 2023 (Term Loan Facility"); bearing interest at LIBOR plus 1.75% \| Secured Debt
Debt Instrument [Line Items]
Principle amount of debt	$ 350,000
Principal amount of debt		$ 350,000
LIBOR \| Revolving Credit Facility due May 2023 ("Revolving Credit Facility"); bearing interest at LIBOR plus 1.75%
Debt Instrument [Line Items]
Basis spread on variable rate	1.75%
LIBOR \| Term Loan A Facility due May 2023 (Term Loan Facility"); bearing interest at LIBOR plus 1.75% \| Secured Debt
Debt Instrument [Line Items]
Basis spread on variable rate	1.75%

Amount of short-term and long-term debt and lease obligation.

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Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.

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Contractual interest rate for funds borrowed, under the debt agreement.

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Weighted average interest rate of debt outstanding.

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Amount, after accumulated amortization, of debt issuance costs. Includes, but is not limited to, legal, accounting, underwriting, printing, and registration costs.

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Present value of lessee's discounted obligation for lease payments from finance lease.

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Weighted average discount rate for finance lease calculated at point in time.

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Amount of long-term debt and lease obligation, classified as noncurrent.

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Amount of long-term debt and lease obligation, classified as current.

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Amount of long-term debt and lease obligation, including portion classified as current.

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