Condensed Consolidating Financial Information - Guarantor Subsidiaries - Condensed Consolidating Balance Sheets (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2014		Dec. 31, 2013		Jun. 30, 2013		Dec. 31, 2012
Current Assets:
Cash	$ 22,617		$ 934		$ 12,190		$ 2,849
Short-term investments	135,796		85,916
Accounts receivable - net	180,492		173,720
Intercompany receivable
Inventories of spare parts - net	73,318		71,137
Prepaid expenses	10,317		8,357
Deferred income taxes	12,723		12,723
Income taxes receivable	1,221		1,871
Total current assets	436,484		354,658
Investment in subsidiaries
Property and equipment - net	822,742		785,472
Restricted investments	15,485		14,685
Other assets	21,794		18,508
Total assets	1,296,505		1,173,323
Current Liabilities:
Accounts payable	33,533		36,874
Accrued and other current liabilities	48,545		54,531
Intercompany payable
Total current liabilities	82,078		91,405
Long-term debt	500,000		379,000
Deferred income taxes and other long-term liabilities	144,831		142,953
Shareholders' Equity:
Common stock and paid-in capital	300,034		298,480
Accumulated other comprehensive loss	6		(24)
Retained earnings	269,556		261,509
Total shareholders' equity	569,596		559,965		527,903		499,615
Total liabilities and shareholders' equity	1,296,505		1,173,323
Parent Company Only (issuer) [Member]
Current Assets:
Cash	2,374		52		1,157		552
Short-term investments	135,796		85,916
Accounts receivable - net	92,572		83,988
Intercompany receivable	156,975		129,026
Inventories of spare parts - net	66,530		71,023
Prepaid expenses	8,096		6,441
Deferred income taxes	12,723		12,723
Income taxes receivable	711		1,361
Total current assets	475,777		390,530
Investment in subsidiaries	125,119		112,273
Property and equipment - net	604,416		579,369
Restricted investments	15,485		14,685
Other assets	22,513		19,111
Total assets	1,243,310		1,115,968
Current Liabilities:
Accounts payable	25,756		31,232
Accrued and other current liabilities	34,048		31,330
Intercompany payable
Total current liabilities	59,804		62,562
Long-term debt	500,000		379,000
Deferred income taxes and other long-term liabilities	113,910		114,441
Shareholders' Equity:
Common stock and paid-in capital	300,034		298,480
Accumulated other comprehensive loss	6		(24)
Retained earnings	269,556		261,509
Total shareholders' equity	569,596		559,965
Total liabilities and shareholders' equity	1,243,310		1,115,968
Guarantor Subsidiaries [Member]
Current Assets:
Cash	20,243	[1]	882	[1]	11,033	[1]	2,297	[1]
Short-term investments		[1]
Accounts receivable - net	87,920	[1]	89,732	[1]
Intercompany receivable		[1]
Inventories of spare parts - net	6,788	[1]	114	[1]
Prepaid expenses	2,221	[1]	1,916	[1]
Deferred income taxes		[1]
Income taxes receivable	510	[1]	510	[1]
Total current assets	117,682	[1]	93,154	[1]
Investment in subsidiaries		[1]
Property and equipment - net	218,326	[1]	206,103	[1]
Restricted investments		[1]
Other assets	(719)	[1]	(603)	[1]
Total assets	335,289	[1]	298,654	[1]
Current Liabilities:
Accounts payable	7,777	[1]	5,642	[1]
Accrued and other current liabilities	14,497	[1]	23,201	[1]
Intercompany payable	156,975	[1]	129,026	[1]
Total current liabilities	179,249	[1]	157,869	[1]
Long-term debt		[1]
Deferred income taxes and other long-term liabilities	30,921	[1]	28,512	[1]
Shareholders' Equity:
Common stock and paid-in capital	2,674	[1]	2,674	[1]
Accumulated other comprehensive loss		[1]
Retained earnings	122,445	[1]	109,599	[1]
Total shareholders' equity	125,119	[1]	112,273	[1]
Total liabilities and shareholders' equity	335,289	[1]	298,654	[1]
Eliminations [Member]
Current Assets:
Cash
Short-term investments
Accounts receivable - net
Intercompany receivable	(156,975)		(129,026)
Inventories of spare parts - net
Prepaid expenses
Deferred income taxes
Income taxes receivable
Total current assets	(156,975)		(129,026)
Investment in subsidiaries	(125,119)		(112,273)
Property and equipment - net
Restricted investments
Other assets
Total assets	(282,094)		(241,299)
Current Liabilities:
Accounts payable
Accrued and other current liabilities
Intercompany payable	(156,975)		(129,026)
Total current liabilities	(156,975)		(129,026)
Long-term debt
Deferred income taxes and other long-term liabilities
Shareholders' Equity:
Common stock and paid-in capital	(2,674)		(2,674)
Accumulated other comprehensive loss
Retained earnings	(122,445)		(109,599)
Total shareholders' equity	(125,119)		(112,273)
Total liabilities and shareholders' equity	$ (282,094)		$ (241,299)

[1] Foreign subsidiaries represent minor subsidiaries and are included in the guarantors' subsidiaries amounts.