Consolidated Statements of Operations - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
REVENUES	$ 440,576	$ 442,845	$ 436,393	$ 445,684	$ 464,134	$ 474,935	$ 463,331	$ 447,385	$ 1,765,498	$ 1,849,785	$ 1,793,087
EXPENSES:
Operating									1,249,537	1,275,586	1,256,128
General and administrative									107,822	107,027	103,936
Depreciation and amortization									147,129	166,746	151,514
Restructuring charges										4,010
Asset impairments									614		955
Costs and Expenses, Total									1,505,102	1,553,369	1,512,533
OPERATING INCOME	65,263	60,815	65,279	69,039	80,359	73,953	77,176	64,928	260,396	296,416	280,554
OTHER (INCOME) EXPENSE:
Interest expense, net									68,535	67,755	49,696
Expenses associated with debt refinancing transactions											701
Other (income) expense									(90)	489	(58)
Total non-operating expense (income)									68,445	68,244	50,339
INCOME BEFORE INCOME TAXES									191,951	228,172	230,215
Income tax expense									(13,911)	(8,253)	(8,361)
NET INCOME	$ 41,340	$ 41,178	$ 45,475	$ 50,047	$ 60,689	$ 55,340	$ 57,583	$ 46,307	$ 178,040	$ 219,919	$ 221,854
BASIC EARNINGS PER SHARE	$ 0.35	$ 0.35	$ 0.38	$ 0.42	$ 0.52	$ 0.47	$ 0.49	$ 0.39	$ 1.51	$ 1.87	$ 1.90
DILUTED EARNINGS PER SHARE	$ 0.35	$ 0.35	$ 0.38	$ 0.42	$ 0.52	$ 0.47	$ 0.49	$ 0.39	1.50	1.87	1.88
DIVIDENDS DECLARED PER SHARE									$ 1.68	$ 2.04	$ 2.16