Consolidated Statements of Operations - USD ($) $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Sep. 30, 2018	Sep. 30, 2017
REVENUES	$ 462,728			$ 442,845			$ 1,353,573	$ 1,324,922
EXPENSES:
Operating	333,759			316,865			973,449	940,065
General and administrative	25,085			28,303			77,594	79,546
Depreciation and amortization	39,465			36,507			116,114	109,564
Asset impairments		$ 1,600		355			1,580	614
Costs and Expenses, Total	398,309			382,030			1,168,737	1,129,789
OPERATING INCOME	64,419			60,815			184,836	195,133
OTHER (INCOME) EXPENSE:
Interest expense, net	20,534			17,029			58,608	50,141
Expenses associated with debt refinancing transactions							1,016
Other (income) expense	49			(65)			39	(108)
Total non-operating expense (income)	20,583			16,964			59,663	50,033
INCOME BEFORE INCOME TAXES	43,836			43,851			125,173	145,100
Income tax expense	(2,842)			(2,673)			(7,205)	(8,400)
NET INCOME	$ 40,994	$ 39,197	$ 37,777	$ 41,178	$ 45,475	$ 50,047	$ 117,968	$ 136,700
BASIC EARNINGS PER SHARE	$ 0.35			$ 0.35			$ 1.00	$ 1.16
DILUTED EARNINGS PER SHARE	$ 0.34			$ 0.35			$ 0.99	$ 1.15