INCOME TAXES - Income tax expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Current:
Federal	$ 314	$ 143	$ 247
State	2,236	1,392	1,634
Total current expense	2,550	1,535	1,881
Deferred:
Federal	8,802	(4,339)	(17,248)
State	(416)	(910)	(8,760)
Total deferred expense (benefit)	8,386	(5,249)	(26,008)
Total income tax expense (benefit)	$ 10,936	$ (3,714)	$ (24,127)
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)	1.60%	10.60%	5.20%
Searchlight investment	(3.30%)
Other permanent differences (as a percent)	2.20%	(4.50%)	(0.90%)
Change in deferred tax rate (as a percent)		(2.90%)	3.70%
Change in deferred tax rate - Federal Tax Reform (as a percent)			6.90%
Valuation allowance (as a percent)	2.80%	(4.70%)	(2.30%)
Provision to return (as a percent)	(1.10%)	(0.50%)	0.50%
Sale of stock in subsidiary (as a percent)			(1.00%)
State audit settlement (as a percent)		(3.20%)
Acquisition related (as a percent)			(1.30%)
Other (as a percent)	(0.50%)	(0.10%)	0.50%
Total (as a percent)	22.70%	15.70%	32.30%
Non-current deferred tax assets:
Reserve for uncollectible accounts	$ 2,420	$ 1,194
Accrued vacation pay deducted when paid	4,354	4,152
Accrued expenses and deferred revenue	16,419	9,839
Net operating loss carryforwards	76,198	86,535
Pension and postretirement obligations	79,688	80,245
Share-based compensation	974	693
Derivative instruments	6,582	5,868
Financing costs	1,177	176
Tax credit carryforwards	4,990	6,077
Total	192,802	194,779
Valuation allowance	(7,139)	(6,680)
Net non-current deferred tax assets	185,663	188,099
Non-current deferred tax liabilities:
Goodwill and other intangibles	(53,797)	(66,271)
Basis in investment	(12)	(5)
Partnership investments	(15,988)	(16,138)
Property, plant and equipment	(286,888)	(278,712)
Other	1
Total	(356,684)	(361,126)
Net non-current deferred taxes	$ (171,021)	$ (173,027)