Condensed Consolidating Financial Information - Condensed Consolidating Statements of Operations (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Condensed Financial Statements, Captions [Line Items]
Operating revenues, net	$ 200,762	$ 170,857	$ 575,273	$ 469,462
Expenses:
Direct expenses	160,731	141,706	468,428	394,496
Selling, general and administrative expenses	9,785	9,784	28,075	28,292
Management fees
Total operating expenses	170,516	151,490	496,503	422,788
Gain on disposal of assets, net	(1,399)	701	(16,075)	11
Impairment of assets			421
Equity in (income) loss of unconsolidated affiliate	(172)		911
Operating income	31,817	18,666	93,513	46,663
Equity in net income of consolidated subsidiaries
Interest expense	7,768	7,488	22,434	22,128
Other income, net	(163)	(262)	(388)	(625)
Total expenses	7,605	7,226	22,046	21,503
Earnings before income taxes	24,212	11,440	71,467	25,160
Income tax expense	10,435	5,056	29,336	10,544
Net earnings	13,777	6,384	42,131	14,616
Parent Company Only (issuer) [Member]
Condensed Financial Statements, Captions [Line Items]
Operating revenues, net	125,160	109,255	354,453	303,623
Expenses:
Direct expenses	98,669	92,980	285,661	260,796
Selling, general and administrative expenses	7,365	7,995	21,719	22,942
Management fees	(3,024)	(2,464)	(8,833)	(6,634)
Total operating expenses	103,010	98,511	298,547	277,104
Gain on disposal of assets, net	(1,380)	701	(16,056)	11
Impairment of assets
Equity in (income) loss of unconsolidated affiliate	(172)		911
Operating income	23,702	10,043	71,051	26,508
Equity in net income of consolidated subsidiaries	(4,787)	(5,174)	(13,395)	(12,086)
Interest expense	7,631	7,488	22,297	22,116
Other income, net	(163)	(262)	(388)	(625)
Total expenses	2,681	2,052	8,514	9,405
Earnings before income taxes	21,021	7,991	62,537	17,103
Income tax expense	7,244	1,607	20,406	2,487
Net earnings	13,777	6,384	42,131	14,616
Guarantor Subsidiaries [Member]
Condensed Financial Statements, Captions [Line Items]
Operating revenues, net	75,602	61,602	220,820	165,839
Expenses:
Direct expenses	62,062	48,726	182,767	133,700
Selling, general and administrative expenses	2,420	1,789	6,356	5,350
Management fees	3,024	2,464	8,833	6,634
Total operating expenses	67,506	52,979	197,956	145,684
Gain on disposal of assets, net	(19)		(19)
Impairment of assets			421
Equity in (income) loss of unconsolidated affiliate
Operating income	8,115	8,623	22,462	20,155
Equity in net income of consolidated subsidiaries
Interest expense	137		137	12
Other income, net
Total expenses	137		137	12
Earnings before income taxes	7,978	8,623	22,325	20,143
Income tax expense	3,191	3,449	8,930	8,057
Net earnings	4,787	5,174	13,395	12,086
Eliminations [Member]
Condensed Financial Statements, Captions [Line Items]
Operating revenues, net
Expenses:
Direct expenses
Selling, general and administrative expenses
Management fees
Total operating expenses
Gain on disposal of assets, net
Impairment of assets
Equity in (income) loss of unconsolidated affiliate
Operating income
Equity in net income of consolidated subsidiaries	4,787	5,174	13,395	12,086
Interest expense
Other income, net
Total expenses	4,787	5,174	13,395	12,086
Earnings before income taxes	(4,787)	(5,174)	(13,395)	(12,086)
Income tax expense
Net earnings	$ (4,787)	$ (5,174)	$ (13,395)	$ (12,086)