LEASES - Maturities of lease liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
LEASES
2021 (excluding the three months ended March 31, 2021)	$ 11,043
Next Twelve Months	14,479	$ 14,877
Year Two	12,086	14,202
Year Three	11,033	11,799
Year Four	8,133	10,633
Thereafter	13,638
Year Five		7,816
Thereafter		13,094
Total lease payments	70,412	72,421
Less imputed interest	(9,619)	(10,097)
Total	$ 60,793	$ 62,324
Operating Lease, Liability, Statement of Financial Position [Extensible List]	atni:LeaseLiabilityCurrent atni:LeaseLiabilityNoncurrent	atni:LeaseLiabilityCurrent atni:LeaseLiabilityNoncurrent
Finance lease liability
2021 (excluding the three months ended March 31, 2021)	$ 377
Next Twelve Months	502	$ 334
Year two	502	333
Year three	380	333
Year four	281	211
Thereafter	522
Total lease payments	2,564	1,211
Less imputed interest	(339)	(82)
Total	$ 2,225	$ 1,129
Finance Lease, Liability, Statement of Financial Position [Extensible List]	atni:LeaseLiabilityCurrent atni:LeaseLiabilityNoncurrent	atni:LeaseLiabilityCurrent atni:LeaseLiabilityNoncurrent