Condensed Consolidating Financial Information - Guarantor Subsidiaries - Condensed Consolidating Statements of Cash Flows (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017		Dec. 31, 2016	Dec. 31, 2015
Condensed Cash Flow Statements, Captions [Line Items]
Net cash (used in) provided by operating activities	$ (19,103)	[1]	$ (564)	$ 133,918
Investing activities:
Purchase of property and equipment	(56,757)		(81,842)	(57,123)
Proceeds from asset dispositions	1,296		14,983	5,236
Purchase of short-term investments	(637,980)		(321,453)	(608,649)
Proceeds from sale of short-term investments	862,942		316,543	505,966
Payments of deposits on aircraft	0		(2,249)	(1,273)
Refund of deposits on aircraft	0		0	6,010
Loan to unconsolidated affiliate	0		(1,200)	0
Loan to third party	(824)		0	0
Business acquisitions net of cash acquired	(126,644)		0	0
Net cash used in investing activities	42,033		(75,218)	(149,833)
Financing activities:
Repurchase of common stock	(256)		(529)	(2,448)
Proceeds from line of credit	152,150		264,700	232,660
Payments on line of credit	(168,650)		(188,200)	(218,160)
Due to/from affiliate, net	0		0	0
Net cash provided by financing activities	(16,756)		75,971	12,052
(Decrease) increase in cash	6,174		189	(3,863)
Cash, beginning of year	2,596		2,407	6,270
Cash, end of year	8,770		2,596	2,407
Parent Company Only (issuer) [Member]
Condensed Cash Flow Statements, Captions [Line Items]
Net cash (used in) provided by operating activities	(62,581)		(46,738)	60,241
Investing activities:
Purchase of property and equipment	(56,757)		(81,484)	(57,123)
Proceeds from asset dispositions	1,296		14,983	5,236
Purchase of short-term investments	(637,980)		(321,453)	(608,649)
Proceeds from sale of short-term investments	862,942		316,543	505,966
Payments of deposits on aircraft	0		(2,249)	(1,273)
Refund of deposits on aircraft				6,010
Loan to unconsolidated affiliate	0		(1,200)
Loan to third party	(824)
Business acquisitions net of cash acquired	(130,788)
Net cash used in investing activities	37,889		(74,860)	(149,833)
Financing activities:
Repurchase of common stock	(256)		(529)	(2,448)
Proceeds from line of credit	152,150		264,700	232,660
Payments on line of credit	(168,650)		(188,200)	(218,160)
Due to/from affiliate, net	41,459		45,617	77,535
Net cash provided by financing activities	24,703		121,588	89,587
(Decrease) increase in cash	11		(10)	(5)
Cash, beginning of year	36		46	51
Cash, end of year	47		36	46
Guarantor Subsidiaries [Member]
Condensed Cash Flow Statements, Captions [Line Items]
Net cash (used in) provided by operating activities	40,628		52,873	78,507
Investing activities:
Purchase of property and equipment	0
Net cash used in investing activities	0		0	0
Financing activities:
Due to/from affiliate, net	(41,656)		(52,445)	(82,801)
Net cash provided by financing activities	(41,656)		(52,445)	(82,801)
(Decrease) increase in cash	(1,028)		428	(4,294)
Cash, beginning of year	2,100		1,672	5,966
Cash, end of year	1,072		2,100	1,672
Non-Guarantor Subsidiaries [Member]
Condensed Cash Flow Statements, Captions [Line Items]
Net cash (used in) provided by operating activities	2,850		(6,699)	(4,830)
Investing activities:
Purchase of property and equipment			(358)
Business acquisitions net of cash acquired	4,144
Net cash used in investing activities	4,144		(358)	0
Financing activities:
Due to/from affiliate, net	197		6,828	5,266
Net cash provided by financing activities	197		6,828	5,266
(Decrease) increase in cash	7,191		(229)	436
Cash, beginning of year	460		689	253
Cash, end of year	7,651		460	689
Eliminations [Member]
Condensed Cash Flow Statements, Captions [Line Items]
Net cash (used in) provided by operating activities	0		0	0
Investing activities:
Net cash used in investing activities	0		0	0
Financing activities:
Net cash provided by financing activities	0		0	0
(Decrease) increase in cash	0		0	0
Cash, beginning of year	0		0	0
Cash, end of year	$ 0		$ 0	$ 0

[1] Net of the effect of business acquisitions.