INCOME TAXES - Income tax expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Current:
Federal	$ 305	$ 314	$ 143
State	470	2,236	1,392
Total current expense	775	2,550	1,535
Deferred:
Federal	4,867	8,802	(4,339)
State	637	(416)	(910)
Total deferred expense (benefit)	5,504	8,386	(5,249)
Total income tax expense (benefit)	$ 6,279	$ 10,936	$ (3,714)
Reconciliation of the provision for income taxes computed at federal statutory rates to the effective rates
Statutory federal income tax rate (as a percent)	21.00%	21.00%	21.00%
State income taxes, net of federal benefit (as a percent)	6.40%	1.60%	10.60%
Searchlight investment (as a percent)	(33.00%)	(3.30%)
Other permanent differences (as a percent)	(0.50%)	2.20%	(4.50%)
Change in deferred tax rate (as a percent)			(2.90%)
Valuation allowance (as a percent)	(1.70%)	2.80%	(4.70%)
Provision to return (as a percent)	2.60%	(1.10%)	(0.50%)
Non deductible goodwill (as a percent)	(1.50%)
State audit settlement (as a percent)			(3.20%)
Other (as a percent)	0.40%	(0.50%)	(0.10%)
Total (as a percent)	(6.30%)	22.70%	15.70%
Non-current deferred tax assets:
Reserve for uncollectible accounts	$ 2,632	$ 2,420
Accrued vacation pay deducted when paid	4,388	4,354
Accrued expenses and deferred revenue	15,019	16,419
Net operating loss carryforwards	100,402	76,198
Excess interest carryforward	2,402
Pension and postretirement obligations	57,507	79,688
Share-based compensation	1,706	974
Derivative instruments	2,633	6,582
Financing costs		1,177
Tax credit carryforwards	4,854	4,990
Total	191,543	192,802
Valuation allowance	(8,580)	(7,139)
Net non-current deferred tax assets	182,963	185,663
Non-current deferred tax liabilities:
Goodwill and other intangibles	(44,044)	(53,797)
Basis in investment	(4)	(12)
Partnership investments	(16,902)	(15,988)
Property, plant and equipment	(310,579)	(286,888)
Financing costs	(5,892)
Other		1
Total	(377,421)	(356,684)
Net non-current deferred taxes	$ (194,458)	$ (171,021)