Consolidated Statements of Operations - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
REVENUES	$ 497,809	$ 508,522	$ 490,294	$ 484,064	$ 482,193	$ 462,728	$ 449,929	$ 440,916	$ 1,980,689	$ 1,835,766	$ 1,765,498
EXPENSES:
Operating									1,422,769	1,315,250	1,249,537
General and administrative									127,078	106,865	107,822
Depreciation and amortization									144,572	156,501	147,129
Contingent consideration for acquisition of businesses										6,085
Asset impairments							1,600		4,706	1,580	614
Costs and Expenses, Total									1,699,125	1,586,281	1,505,102
OPERATING INCOME	66,251	71,095	70,954	73,264	64,649	64,419	61,712	58,705	281,564	249,485	260,396
OTHER (INCOME) EXPENSE:
Interest expense, net									84,401	80,753	68,535
Expenses associated with debt refinancing transactions									602	1,016
Other (income) expense									(164)	156	(90)
Total non-operating expense (income)									84,839	81,925	68,445
INCOME BEFORE INCOME TAXES									196,725	167,560	191,951
Income tax expense									(7,839)	(8,353)	(13,911)
NET INCOME	$ 41,974	$ 48,994	$ 48,578	$ 49,340	$ 41,239	$ 40,994	$ 39,197	$ 37,777	$ 188,886	$ 159,207	$ 178,040
BASIC EARNINGS PER SHARE	$ 0.35	$ 0.41	$ 0.41	$ 0.42	$ 0.35	$ 0.35	$ 0.33	$ 0.32	$ 1.59	$ 1.34	$ 1.51
DILUTED EARNINGS PER SHARE	$ 0.35	$ 0.41	$ 0.41	$ 0.41	$ 0.35	$ 0.34	$ 0.33	$ 0.32	1.59	1.34	1.50
DIVIDENDS DECLARED PER SHARE									$ 1.76	$ 1.72	$ 1.68