Condensed Consolidating Financial Information - Guarantor Subsidiaries - Condensed Consolidating Balance Sheets (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012		Dec. 31, 2011
Current Assets:
Cash	$ 6,270		$ 934		$ 2,849		$ 5,091
Short-term investments	185,244		85,916
Accounts receivable - net	180,761		173,720
Inventories of spare parts - net	73,793		71,137
Prepaid expenses	9,314		8,357
Deferred income taxes	9,915		12,723
Income taxes receivable	1,227		1,871
Total current assets	466,524		354,658
Property and equipment, net	877,818		785,472
Restricted investments	15,485		14,685
Other assets	16,253		18,508
Total assets	1,376,080		1,173,323
Current Liabilities:
Accounts payable	27,700		36,874
Accrued and other current liabilities	52,812		54,531
Total current liabilities	80,512		91,405
Long-term debt	543,000		379,000
Deferred income taxes and other long-term liabilities	155,500		142,953
Shareholders' Equity:
Common stock and paid-in capital	303,082		298,480
Accumulated other comprehensive loss	(211)		(24)
Retained earnings	294,197		261,509
Total shareholders' equity	597,068		559,965		499,615		477,337
Total liabilities and shareholders' equity	1,376,080		1,173,323
Parent Company Only (issuer) [Member]
Current Assets:
Cash	51		52		552		4,313
Short-term investments	185,244		85,916
Accounts receivable - net	98,001		83,988
Intercompany receivable			129,026
Inventories of spare parts - net	65,341		71,023
Prepaid expenses	7,610		6,441
Deferred income taxes	9,915		12,723
Income taxes receivable	1,068		1,361
Total current assets	367,230		390,530
Investment in subsidiaries	358,080		112,273
Property and equipment, net	638,437		579,369
Restricted investments	15,485		14,685
Other assets	16,055		19,111
Total assets	1,395,287		1,115,968
Current Liabilities:
Accounts payable	22,578		31,232
Accrued and other current liabilities	34,477		31,330
Intercompany payable	95,270
Total current liabilities	152,325		62,562
Long-term debt	543,000		379,000
Deferred income taxes and other long-term liabilities	102,894		114,441
Shareholders' Equity:
Common stock and paid-in capital	303,082		298,480
Accumulated other comprehensive loss	(211)		(24)
Retained earnings	294,197		261,509
Total shareholders' equity	597,068		559,965
Total liabilities and shareholders' equity	1,395,287		1,115,968
Guarantor Subsidiaries [Member]
Current Assets:
Cash	6,219	[1],[2]	882	[1],[2]	2,297	[1],[2]	778	[1],[2]
Accounts receivable - net	82,760	[1]	89,732	[1]
Intercompany receivable	95,399	[1]
Inventories of spare parts - net	8,452	[1]	114	[1]
Prepaid expenses	1,704	[1]	1,916	[1]
Income taxes receivable	159	[1]	510	[1]
Total current assets	194,693	[1]	93,154	[1]
Property and equipment, net	239,381	[1]	206,103	[1]
Other assets	198	[1]	(603)	[1]
Total assets	434,272	[1]	298,654	[1]
Current Liabilities:
Accounts payable	5,122	[1]	5,642	[1]
Accrued and other current liabilities	18,335	[1]	23,201	[1]
Intercompany payable			129,026	[1]
Total current liabilities	23,457	[1]	157,869	[1]
Deferred income taxes and other long-term liabilities	52,606	[1]	28,512	[1]
Shareholders' Equity:
Common stock and paid-in capital	137,647	[1]	2,674	[1]
Retained earnings	220,562	[1]	109,599	[1]
Total shareholders' equity	358,209	[1]	112,273	[1]
Total liabilities and shareholders' equity	434,272	[1]	298,654	[1]
Eliminations [Member]
Current Assets:
Intercompany receivable	(95,399)		(129,026)
Total current assets	(95,399)		(129,026)
Investment in subsidiaries	(358,080)		(112,273)
Total assets	(453,479)		(241,299)
Current Liabilities:
Intercompany payable	(95,270)		(129,026)
Total current liabilities	(95,270)		(129,026)
Shareholders' Equity:
Common stock and paid-in capital	(137,647)		(2,674)
Retained earnings	(220,562)		(109,599)
Total shareholders' equity	(358,209)		(112,273)
Total liabilities and shareholders' equity	$ (453,479)		$ (241,299)

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

[2] Cash flows between the Parent Company and Guarantor Subsidiaries related to the Company's centralized cash management activities in 2013 and 2012 have been adjusted to show the effects on net cash provided by operating and financing activities.