Condensed Consolidating Financial Information - Guarantor Subsidiaries - Condensed Consolidating Balance Sheets (Detail) - USD ($) $ in Thousands		Jun. 30, 2016	Dec. 31, 2015	Jun. 30, 2015	Dec. 31, 2014
Current Assets:
Cash		$ 8,567	$ 2,407	$ 3,659	$ 6,270
Short-term investments		289,235	284,523
Accounts receivable - net		148,412	144,778
Inventories of spare parts - net		69,424	69,491
Prepaid expenses		7,802	8,951
Deferred income taxes		10,379	10,379
Income taxes receivable		709	761
Total current assets		534,528	521,290
Property and equipment - net		893,235	883,529
Restricted investments		13,038	15,336
Other assets		6,317	6,178
Total assets		1,447,118	1,426,333
Current Liabilities:
Accounts payable		23,462	31,373
Accrued and other current liabilities		40,343	44,759
Total current liabilities		63,805	76,132
Long-term debt:
Revolving credit facility		95,000	57,500
Senior Notes dated March 17, 2014, net of debt issuance costs of $3,376		496,624	496,001
Deferred income taxes and other long-term liabilities		166,343	169,702
Shareholders' Equity:
Common stock and paid-in capital		308,835	306,444
Accumulated other comprehensive income (loss)		45	(567)
Retained earnings		316,466	321,121
Total shareholders' equity		625,346	626,998	614,920	597,068
Total liabilities and shareholders' equity		1,447,118	1,426,333
Parent Company Only (issuer) [Member]
Current Assets:
Cash		40	46	48	51
Short-term investments		289,235	284,523
Accounts receivable - net		64,631	70,336
Inventories of spare parts - net		60,768	60,060
Prepaid expenses		4,821	7,162
Deferred income taxes		10,379	10,379
Income taxes receivable		950	1,002
Total current assets		430,824	433,508
Investment in subsidiaries		343,068	330,848
Property and equipment - net		598,361	632,759
Restricted investments		13,023	15,336
Other assets		5,191	5,975
Total assets		1,390,467	1,418,426
Current Liabilities:
Accounts payable		16,742	25,512
Accrued and other current liabilities		28,825	29,138
Intercompany payable		47,588	90,943
Total current liabilities		93,155	145,593
Long-term debt:
Revolving credit facility		95,000	57,500
Senior Notes dated March 17, 2014, net of debt issuance costs of $3,376		496,624	496,001
Deferred income taxes and other long-term liabilities		80,342	92,334
Shareholders' Equity:
Common stock and paid-in capital		308,835	306,444
Accumulated other comprehensive income (loss)		45	(567)
Retained earnings		316,466	321,121
Total shareholders' equity		625,346	626,998
Total liabilities and shareholders' equity		1,390,467	1,418,426
Guarantor Subsidiaries [Member]
Current Assets:
Cash	[1]	8,527	2,361	$ 3,611	$ 6,219
Accounts receivable - net	[1]	83,781	74,442
Intercompany receivable	[1]	47,588	90,943
Inventories of spare parts - net	[1]	8,656	9,431
Prepaid expenses	[1]	2,981	1,789
Income taxes receivable	[1]	(241)	(241)
Total current assets	[1]	151,292	178,725
Property and equipment - net	[1]	294,874	250,770
Restricted investments	[1]	15
Other assets	[1]	1,126	203
Total assets	[1]	447,307	429,698
Current Liabilities:
Accounts payable	[1]	6,720	5,861
Accrued and other current liabilities	[1]	11,518	15,621
Total current liabilities	[1]	18,238	21,482
Long-term debt:
Deferred income taxes and other long-term liabilities	[1]	86,001	77,368
Shareholders' Equity:
Common stock and paid-in capital	[1]	79,191	79,061
Retained earnings	[1]	263,877	251,787
Total shareholders' equity	[1]	343,068	330,848
Total liabilities and shareholders' equity	[1]	447,307	429,698
Eliminations [Member]
Current Assets:
Intercompany receivable		(47,588)	(90,943)
Total current assets		(47,588)	(90,943)
Investment in subsidiaries		(343,068)	(330,848)
Total assets		(390,656)	(421,791)
Current Liabilities:
Intercompany payable		(47,588)	(90,943)
Total current liabilities		(47,588)	(90,943)
Shareholders' Equity:
Common stock and paid-in capital		(79,191)	(79,061)
Retained earnings		(263,877)	(251,787)
Total shareholders' equity		(343,068)	(330,848)
Total liabilities and shareholders' equity		$ (390,656)	$ (421,791)

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