Condensed Consolidating Financial Information - Condensed Consolidating Balance Sheets (Detail) - USD ($) $ in Thousands		Jun. 30, 2015	Dec. 31, 2014	Jun. 30, 2014	Dec. 31, 2013
Current Assets:
Cash		$ 3,659	$ 6,270	$ 22,617	$ 934
Short-term investments		217,071	185,244
Accounts receivable - net		172,523	180,761
Inventories of spare parts - net		70,320	73,793
Prepaid expenses		11,836	9,314
Deferred income taxes		9,915	9,915
Income taxes receivable		1,227	1,227
Total current assets		486,551	466,524
Property and equipment - net		911,112	877,818
Restricted investments		15,336	15,485
Other assets		14,852	16,253
Total assets		1,427,851	1,376,080
Current Liabilities:
Accounts payable		63,134	27,700
Accrued and other current liabilities		48,550	52,812
Total current liabilities		111,684	80,512
Long-term debt		535,500	543,000
Deferred income taxes and other long-term liabilities		165,747	155,500
Shareholders' Equity:
Common stock and paid-in capital		304,103	303,082
Accumulated other comprehensive loss		(197)	(211)
Retained earnings		311,014	294,197
Total shareholders' equity		614,920	597,068	569,596	559,965
Total liabilities and shareholders' equity		1,427,851	1,376,080
Parent Company Only (issuer) [Member]
Current Assets:
Cash		48	51	2,374	52
Short-term investments		217,071	185,244
Accounts receivable - net		86,298	98,001
Inventories of spare parts - net		61,404	65,341
Prepaid expenses		8,837	7,610
Deferred income taxes		9,915	9,915
Income taxes receivable		1,068	1,068
Total current assets		384,641	367,230
Investment in subsidiaries		348,123	358,080
Property and equipment - net		676,478	638,437
Restricted investments		15,336	15,485
Other assets		14,648	16,055
Total assets		1,439,226	1,395,287
Current Liabilities:
Accounts payable		58,387	22,578
Accrued and other current liabilities		34,024	34,477
Intercompany payable		93,863	95,270
Total current liabilities		186,274	152,325
Long-term debt		535,500	543,000
Deferred income taxes and other long-term liabilities		102,532	102,894
Shareholders' Equity:
Common stock and paid-in capital		304,103	303,082
Accumulated other comprehensive loss		(197)	(211)
Retained earnings		311,014	294,197
Total shareholders' equity		614,920	597,068
Total liabilities and shareholders' equity		1,439,226	1,395,287
Guarantor Subsidiaries [Member]
Current Assets:
Cash	[1]	3,611	6,219	$ 20,243	$ 882
Accounts receivable - net	[1]	86,225	82,760
Intercompany receivable	[1]	93,863	95,399
Inventories of spare parts - net	[1]	8,916	8,452
Prepaid expenses	[1]	2,999	1,704
Income taxes receivable	[1]	159	159
Total current assets	[1]	195,773	194,693
Property and equipment - net	[1]	234,634	239,381
Other assets	[1]	204	198
Total assets	[1]	430,611	434,272
Current Liabilities:
Accounts payable	[1]	4,747	5,122
Accrued and other current liabilities	[1]	14,526	18,335
Total current liabilities	[1]	19,273	23,457
Deferred income taxes and other long-term liabilities	[1]	63,215	52,606
Shareholders' Equity:
Common stock and paid-in capital	[1]	112,199	137,647
Retained earnings	[1]	235,924	220,562
Total shareholders' equity	[1]	348,123	358,209
Total liabilities and shareholders' equity	[1]	430,611	434,272
Eliminations [Member]
Current Assets:
Intercompany receivable		(93,863)	(95,399)
Total current assets		(93,863)	(95,399)
Investment in subsidiaries		(348,123)	(358,080)
Total assets		(441,986)	(453,479)
Current Liabilities:
Intercompany payable		(93,863)	(95,270)
Total current liabilities		(93,863)	(95,270)
Shareholders' Equity:
Common stock and paid-in capital		(112,199)	(137,647)
Retained earnings		(235,924)	(220,562)
Total shareholders' equity		(348,123)	(358,209)
Total liabilities and shareholders' equity		$ (441,986)	$ (453,479)

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