Condensed Consolidating Financial Information - Condensed Consolidating Balance Sheets (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015		Dec. 31, 2014		Mar. 31, 2014		Dec. 31, 2013
Current Assets:
Cash	$ 3,623		$ 6,270		$ 9,336		$ 934
Short-term investments	189,480		185,244
Accounts receivable - net	160,227		180,761
Inventories of spare parts - net	71,452		73,793
Prepaid expenses	11,075		9,314
Deferred income taxes	9,914		9,915
Income taxes receivable	1,226		1,227
Total current assets	446,997		466,524
Property and equipment - net	917,342		877,818
Restricted investments	15,485		15,485
Other assets	15,095		16,253
Total assets	1,394,919		1,376,080
Current Liabilities:
Accounts payable	56,214		27,700
Accrued and other current liabilities	30,085		52,812
Total current liabilities	86,299		80,512
Long-term debt	539,640		543,000
Deferred income taxes and other long-term liabilities	161,929		155,500
Shareholders' Equity:
Common stock and paid-in capital	302,593		303,082
Accumulated other comprehensive loss	(102)		(211)
Retained earnings	304,560		294,197
Total shareholders' equity	607,051		597,068		555,165		559,965
Total liabilities and shareholders' equity	1,394,919		1,376,080
Parent Company Only (issuer) [Member]
Current Assets:
Cash	51		51		7,379		52
Short-term investments	189,480		185,244
Accounts receivable - net	100,167		98,001
Inventories of spare parts - net	62,954		65,341
Prepaid expenses	8,475		7,610
Deferred income taxes	9,914		9,915
Income taxes receivable	1,067		1,068
Total current assets	372,108		367,230
Investment in subsidiaries	336,676		358,080
Property and equipment - net	680,419		638,437
Restricted investments	15,485		15,485
Other assets	14,891		16,055
Total assets	1,419,579		1,395,287
Current Liabilities:
Accounts payable	51,344		22,578
Accrued and other current liabilities	21,233		34,477
Intercompany payable	95,103		95,270
Total current liabilities	167,680		152,325
Long-term debt	539,640		543,000
Deferred income taxes and other long-term liabilities	105,208		102,894
Shareholders' Equity:
Common stock and paid-in capital	302,593		303,082
Accumulated other comprehensive loss	(102)		(211)
Retained earnings	304,560		294,197
Total shareholders' equity	607,051		597,068
Total liabilities and shareholders' equity	1,419,579		1,395,287
Guarantor Subsidiaries [Member]
Current Assets:
Cash	3,572	[1]	6,219	[1]	1,957	[1],[2]	882	[1],[2]
Accounts receivable - net	60,060	[1]	82,760	[1]
Intercompany receivable	95,103	[1]	95,399	[1]
Inventories of spare parts - net	8,498	[1]	8,452	[1]
Prepaid expenses	2,600	[1]	1,704	[1]
Income taxes receivable	159	[1]	159	[1]
Total current assets	169,992	[1]	194,693	[1]
Property and equipment - net	236,923	[1]	239,381	[1]
Other assets	204	[1]	198	[1]
Total assets	407,119	[1]	434,272	[1]
Current Liabilities:
Accounts payable	4,870	[1]	5,122	[1]
Accrued and other current liabilities	8,852	[1]	18,335	[1]
Total current liabilities	13,722	[1]	23,457	[1]
Deferred income taxes and other long-term liabilities	56,721	[1]	52,606	[1]
Shareholders' Equity:
Common stock and paid-in capital	109,982	[1]	137,647	[1]
Retained earnings	226,694	[1]	220,562	[1]
Total shareholders' equity	336,676	[1]	358,209	[1]
Total liabilities and shareholders' equity	407,119	[1]	434,272	[1]
Eliminations [Member]
Current Assets:
Intercompany receivable	(95,103)		(95,399)
Total current assets	(95,103)		(95,399)
Investment in subsidiaries	(336,676)		(358,080)
Total assets	(431,779)		(453,479)
Current Liabilities:
Intercompany payable	(95,103)		(95,270)
Total current liabilities	(95,103)		(95,270)
Shareholders' Equity:
Common stock and paid-in capital	(109,982)		(137,647)
Retained earnings	(226,694)		(220,562)
Total shareholders' equity	(336,676)		(358,209)
Total liabilities and shareholders' equity	$ (431,779)		$ (453,479)

[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 2014 have been adjusted to show the effects on net cash provided by operating and financing activities.