QUARTERLY RESULTS OF OPERATIONS (UNAUDITED) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Revenues	$ 816,414	$ 794,980	$ 627,370	$ 701,399	$ 717,418	$ 787,563	$ 771,490	$ 711,839	$ 2,940,163	$ 2,988,310	$ 3,093,788
Operating expenses	786,162	755,198	606,945	693,580	728,647	756,355	736,290	703,248	2,841,885	2,924,540	2,984,690
OPERATING INCOME	30,252	39,782	20,425	7,819	(11,229)	31,208	35,200	8,591	98,278	63,770	109,098
Other income (costs)	(53)	(604)	309	(5,434)	(798)	(7,866)	(1,640)	(1,995)	(5,782)	(12,299)	(24,712)
Income tax provision (benefit)	6,285	9,774	4,854	483	(6,478)	7,072	9,184	1,708	21,396	11,486	17,124
NET INCOME	$ 23,914	$ 29,404	$ 15,880	$ 1,902	$ (5,549)	$ 16,270	$ 24,376	$ 4,888	$ 71,100	$ 39,985	$ 67,262
Earnings (loss) per common share
Basic (in dollars per share)	$ 0.94	$ 1.15	$ 0.62	$ 0.07	$ (0.22)	$ 0.64	$ 0.95	$ 0.19	$ 2.80	$ 1.56	$ 2.61
Diluted (in dollars per share)	$ 0.89	$ 1.11	$ 0.61	$ 0.07	$ (0.22)	$ 0.62	$ 0.92	$ 0.18	$ 2.69	$ 1.51	$ 2.51
AVERAGE COMMON SHARES OUTSTANDING
Basic (in shares)	25,427,449	25,470,094	25,463,559	25,390,377	25,490,393	25,527,982	25,554,286	25,570,415	25,410,232	25,535,529	25,679,736
Diluted (in shares)	26,734,287	26,592,457	26,217,957	26,246,800	25,490,393	26,416,595	26,431,592	26,512,349	26,422,523	26,450,055	26,698,831
Goodwill and Intangible Asset Impairment [Abstract]
Noncash impairment charge (pre-tax)					$ 26,500					$ 26,514
Noncash impairment charge (after-tax)					$ 19,800
Noncash impairment charge diluted (in dollars per share)					$ 0.78
Nonunion Defined Benefit Pension Plan
Nonunion pension expense
Nonunion pension expense, including settlement and termination expense (pre-tax)						$ 6,700
Nonunion pension expense, including settlement and termination expense (after-tax)						$ 6,000
Nonunion pension expense per diluted share, including settlement and termination expense (in dollars per share)						$ 0.23