Condensed Consolidating Financial Information - Guarantor Subsidiaries - Condensed Consolidating Statements of Cash Flows (Detail) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Condensed Cash Flow Statements, Captions [Line Items]
Net cash (used in) provided by operating activities		$ 5,410	$ (7,949)
Investing activities:
Purchase of property and equipment		(18,098)	(43,892)
Proceeds from asset dispositions		1,453	17
Purchase of short-term investments		(260,996)	(134,518)
Proceeds from sale of short-term investments		264,918	187,217
Payments of deposits on aircraft		0	(110)
Loan to unconsolidated affiliate		(274)	0
Net cash provided by (used in) investing activities		(12,997)	8,714
Debt issuance cost		0
Financing activities:
Proceeds from line of credit		34,295	66,525
Payments on line of credit		(29,575)	(67,300)
Repurchase of common stock		(51)	(263)
Net cash provided by (used in) financing activities		4,669	(1,038)
Increase (decrease) in cash		(2,918)	(273)
Cash, beginning of period		8,770	2,596
Cash, end of period	$ 2,323	5,852	2,323
Parent Company Only (issuer) [Member]
Condensed Cash Flow Statements, Captions [Line Items]
Net cash (used in) provided by operating activities		(506)	(34,236)
Investing activities:
Purchase of property and equipment		(16,768)	(43,892)
Proceeds from asset dispositions		1,453	17
Purchase of short-term investments		(260,996)	(134,518)
Proceeds from sale of short-term investments		264,918	187,217
Payments of deposits on aircraft			(110)
Loan to unconsolidated affiliate		(274)
Net cash provided by (used in) investing activities		(11,667)	8,714
Debt issuance cost		0
Financing activities:
Proceeds from line of credit		34,295	66,525
Payments on line of credit		(29,575)	(67,300)
Repurchase of common stock		(51)	(263)
Due to/from affiliate, net		7,510	26,572
Net cash provided by (used in) financing activities		12,179	25,534
Increase (decrease) in cash		6	12
Cash, beginning of period		47	36
Cash, end of period	48	53	48
Guarantor Subsidiaries [Member]
Condensed Cash Flow Statements, Captions [Line Items]
Net cash (used in) provided by operating activities		3,306	18,916
Investing activities:
Purchase of property and equipment			0
Net cash provided by (used in) investing activities		0	0
Financing activities:
Due to/from affiliate, net		(3,073)	(19,688)
Net cash provided by (used in) financing activities		(3,073)	(19,688)
Increase (decrease) in cash		233	(772)
Cash, beginning of period		1,072	2,100
Cash, end of period	1,328	1,305	1,328
Non-Guarantor Subsidiaries [Member]
Condensed Cash Flow Statements, Captions [Line Items]
Net cash (used in) provided by operating activities	7,371	2,610
Investing activities:
Purchase of property and equipment	0	(1,330)
Net cash provided by (used in) investing activities	0	(1,330)
Financing activities:
Due to/from affiliate, net	(6,884)	(4,437)
Net cash provided by (used in) financing activities	(6,884)	(4,437)
Increase (decrease) in cash	487	(3,157)
Cash, beginning of period		7,651	460
Cash, end of period	$ 947	$ 4,494	$ 947