Consolidated Statements of Operations - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
REVENUES	$ 482,193	$ 462,728	$ 449,929	$ 440,916	$ 440,576	$ 442,845	$ 436,393	$ 445,684	$ 1,835,766	$ 1,765,498	$ 1,849,785
EXPENSES:
Operating									1,315,250	1,249,537	1,275,586
General and administrative									106,865	107,822	107,027
Depreciation and amortization									156,501	147,129	166,746
Contingent consideration for acquisition of businesses									6,085
Restructuring charges											4,010
Asset impairments			1,600						1,580	614
Costs and Expenses, Total									1,586,281	1,505,102	1,553,369
OPERATING INCOME	64,649	64,419	61,712	58,705	65,263	60,815	65,279	69,039	249,485	260,396	296,416
OTHER (INCOME) EXPENSE:
Interest expense, net									80,753	68,535	67,755
Expenses associated with debt refinancing transactions									1,016
Other (income) expense									156	(90)	489
Total non-operating expense (income)									81,925	68,445	68,244
INCOME BEFORE INCOME TAXES									167,560	191,951	228,172
Income tax expense									(8,353)	(13,911)	(8,253)
NET INCOME	$ 41,239	$ 40,994	$ 39,197	$ 37,777	$ 41,340	$ 41,178	$ 45,475	$ 50,047	$ 159,207	$ 178,040	$ 219,919
BASIC EARNINGS PER SHARE	$ 0.35	$ 0.35	$ 0.33	$ 0.32	$ 0.35	$ 0.35	$ 0.38	$ 0.42	$ 1.34	$ 1.51	$ 1.87
DILUTED EARNINGS PER SHARE	$ 0.35	$ 0.34	$ 0.33	$ 0.32	$ 0.35	$ 0.35	$ 0.38	$ 0.42	1.34	1.50	1.87
DIVIDENDS DECLARED PER SHARE									$ 1.72	$ 1.68	$ 2.04