DEBT - Summary of Debt (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2021	Dec. 31, 2020
Debt Instrument [Line Items]
Weighted-average discount rate - finance leases	4.00%
Finance leases maturing through December 2107; bearing interest at a weighted average of 4.0%	$ 34,573	$ 31,486
Principal amount of debt	550,220	548,419
Less - unamortized debt issuance costs	8,197	8,768
Debt less unamortized debt issuance costs	542,023	539,651
Less—current maturities of debt	10,918	9,240
Debt, less current portion	531,105	530,411
Revolving line of credit facility ("Revolving Credit Facility") due May 2023; bearing interest at LIBOR plus 1.75%
Debt Instrument [Line Items]
Principal amount of debt	$ 0	0
Revolving line of credit facility ("Revolving Credit Facility") due May 2023; bearing interest at LIBOR plus 1.75% \| LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate	1.75%
Secured Debt \| Term loan A facility ("Term Loan Facility") due May 2023; bearing interest at LIBOR plus 1.75%
Debt Instrument [Line Items]
Principal amount of debt	$ 349,125	350,000
Secured Debt \| Term loan A facility ("Term Loan Facility") due May 2023; bearing interest at LIBOR plus 1.75% \| LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate	1.75%
Credit Facility
Debt Instrument [Line Items]
Less - unamortized debt issuance costs	$ 3,429	3,839
Unsecured Debt \| New York State Environmental Facilities Corporation Solid Waste Disposal Revenue Bonds Series 2014 ("New York Bonds 2014R-1") due December 2044 - fixed rate interest period through 2029; bearing interest at 2.875%
Debt Instrument [Line Items]
Stated interest rate	2.875%
Principal amount of debt	$ 25,000	25,000
Less - unamortized debt issuance costs	$ 984	1,000
Unsecured Debt \| 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]
Stated interest rate	3.125%
Principal amount of debt	$ 15,000	15,000
Less - unamortized debt issuance costs	$ 314	329
Unsecured Debt \| New York State Environmental Facilities Corporation Solid Waste Disposal Revenue Bonds Series 2020 ("New York Bonds 2020") due September 2050 - fixed rate interest period through 2025; bearing interest at 2.750%
Debt Instrument [Line Items]
Stated interest rate	2.75%
Principal amount of debt	$ 40,000	40,000
Less - unamortized debt issuance costs	$ 1,417	1,461
Unsecured Debt \| 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]
Stated interest rate	5.25%
Principal amount of debt	$ 25,000	25,000
Less - unamortized debt issuance costs	$ 325	347
Unsecured Debt \| 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]
Stated interest rate	5.125%
Principal amount of debt	$ 15,000	15,000
Less - unamortized debt issuance costs	$ 465	482
Unsecured Debt \| 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]
Stated interest rate	4.375%
Principal amount of debt	$ 15,000	15,000
Less - unamortized debt issuance costs	$ 324	343
Unsecured Debt \| 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]
Stated interest rate	4.625%
Principal amount of debt	$ 16,000	16,000
Less - unamortized debt issuance costs	$ 473	487
Unsecured Debt \| 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]
Stated interest rate	2.95%
Principal amount of debt	$ 11,000	11,000
Less - unamortized debt issuance costs	$ 466	480
Notes payable maturing through June 2027; bearing interest at a weighted average of 3.4%
Debt Instrument [Line Items]
Weighted average interest rate	3.40%
Principal amount of debt	$ 4,522	$ 4,933

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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