Condensed Consolidating Statement of Operations (Unaudited) (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Condensed Financial Statements, Captions [Line Items]
REVENUES	$ 484,064	$ 440,916
EXPENSES:
Operating	345,832	319,151
General and administrative	29,445	24,971
Depreciation and amortization	35,523	38,089
Costs and Expenses, Total	410,800	382,211
OPERATING INCOME	73,264	58,705
OTHER (INCOME) EXPENSE:
Interest expense, net	21,436	19,036
Other (income) expense	4	(43)
Total non-operating expense (income)	21,440	18,993
INCOME BEFORE INCOME TAXES	51,824	39,712
Income tax expense	(2,484)	(1,935)
INCOME BEFORE EQUITY IN SUBSIDIARIES	49,340	37,777
NET INCOME	49,340	37,777
Reportable Legal Entities | Parent
Condensed Financial Statements, Captions [Line Items]
REVENUES	335,212	307,529
EXPENSES:
Operating	257,493	242,185
General and administrative	10,028	8,235
Depreciation and amortization	23,284	22,533
Costs and Expenses, Total	290,805	272,953
OPERATING INCOME	44,407	34,576
OTHER (INCOME) EXPENSE:
Interest expense, net	18,286	16,177
Other (income) expense	(68)	(118)
Total non-operating expense (income)	18,218	16,059
INCOME BEFORE INCOME TAXES	26,189	18,517
Income tax expense	(529)	(423)
INCOME BEFORE EQUITY IN SUBSIDIARIES	25,660	18,094
Income from equity in subsidiaries	23,680	19,683
NET INCOME	49,340	37,777
Reportable Legal Entities | Combined Subsidiary Guarantors
Condensed Financial Statements, Captions [Line Items]
REVENUES	392,077	366,685
EXPENSES:
Operating	334,995	310,264
General and administrative	19,417	16,736
Depreciation and amortization	10,315	15,556
Costs and Expenses, Total	364,727	342,556
OPERATING INCOME	27,350	24,129
OTHER (INCOME) EXPENSE:
Interest expense, net	1,453	2,859
Other (income) expense	40	58
Total non-operating expense (income)	1,493	2,917
INCOME BEFORE INCOME TAXES	25,857	21,212
Income tax expense	(1,955)	(1,512)
INCOME BEFORE EQUITY IN SUBSIDIARIES	23,902	19,700
NET INCOME	23,902	19,700
Reportable Legal Entities | Non-Guarantor Subsidiaries
Condensed Financial Statements, Captions [Line Items]
REVENUES	5,723
EXPENSES:
Operating	2,292
Depreciation and amortization	1,924
Costs and Expenses, Total	4,216
OPERATING INCOME	1,507
OTHER (INCOME) EXPENSE:
Interest expense, net	1,697
Other (income) expense	32
Total non-operating expense (income)	1,729
INCOME BEFORE INCOME TAXES	(222)
INCOME BEFORE EQUITY IN SUBSIDIARIES	(222)
NET INCOME	(222)
Consolidating Adjustments and Other
Condensed Financial Statements, Captions [Line Items]
REVENUES	(248,948)	(233,298)
EXPENSES:
Operating	(248,948)	(233,298)
Costs and Expenses, Total	(248,948)	(233,298)
OTHER (INCOME) EXPENSE:
Other (income) expense		17
Total non-operating expense (income)		17
INCOME BEFORE INCOME TAXES		(17)
INCOME BEFORE EQUITY IN SUBSIDIARIES		(17)
Income from equity in subsidiaries	(23,680)	(19,683)
NET INCOME	$ (23,680)	$ (19,700)