Condensed Consolidating Financial Information - Guarantor Subsidiaries - Condensed Consolidating Balance Sheets (Detail) - USD ($) $ in Thousands		Sep. 30, 2016	Dec. 31, 2015	Sep. 30, 2015	Dec. 31, 2014
Current Assets:
Cash		$ 2,639	$ 2,407	$ 5,552	$ 6,270
Short-term investments		289,520	284,523
Accounts receivable - net		142,475	144,778
Inventories of spare parts - net		69,409	69,491
Prepaid expenses		6,976	8,951
Deferred income taxes		10,379	10,379
Income taxes receivable		863	761
Total current assets		522,261	521,290
Property and equipment - net		916,560	883,529
Restricted cash and investments		13,038	15,336
Other assets		7,334	6,178
Total assets		1,459,193	1,426,333
Current Liabilities:
Accounts payable		20,843	31,373
Accrued and other current liabilities		32,178	44,759
Total current liabilities		53,021	76,132
Long-term debt:
Revolving credit facility		132,400	57,500
Senior Notes dated March 17, 2014, net of debt issuance costs of $3,064		496,936	496,001
Deferred income taxes and other long-term liabilities		155,349	169,702
Shareholders' Equity:
Common stock and paid-in capital		310,259	306,444
Accumulated other comprehensive income (loss)		(271)	(567)
Retained earnings		311,499	321,121
Total shareholders' equity		621,487	626,998	624,195	597,068
Total liabilities and shareholders' equity		1,459,193	1,426,333
Parent Company Only (issuer) [Member]
Current Assets:
Cash		36	46	1,460	51
Short-term investments		289,520	284,523
Accounts receivable - net		64,564	70,336
Inventories of spare parts - net		60,416	60,060
Prepaid expenses		4,412	7,162
Deferred income taxes		10,379	10,379
Income taxes receivable		950	1,002
Total current assets		430,277	433,508
Investment in subsidiaries		351,440	330,848
Property and equipment - net		602,296	632,759
Restricted cash and investments		13,023	15,336
Other assets		6,181	5,975
Total assets		1,403,217	1,418,426
Current Liabilities:
Accounts payable		17,232	25,512
Accrued and other current liabilities		21,894	29,138
Intercompany payable		41,796	90,943
Total current liabilities		80,922	145,593
Long-term debt:
Revolving credit facility		132,400	57,500
Senior Notes dated March 17, 2014, net of debt issuance costs of $3,064		496,936	496,001
Deferred income taxes and other long-term liabilities		71,472	92,334
Shareholders' Equity:
Common stock and paid-in capital		310,259	306,444
Accumulated other comprehensive income (loss)		(271)	(567)
Retained earnings		311,499	321,121
Total shareholders' equity		621,487	626,998
Total liabilities and shareholders' equity		1,403,217	1,418,426
Guarantor Subsidiaries [Member]
Current Assets:
Cash	[1]	2,603	2,361	$ 4,092	$ 6,219
Accounts receivable - net	[1]	77,911	74,442
Intercompany receivable	[1]	41,796	90,943
Inventories of spare parts - net	[1]	8,993	9,431
Prepaid expenses	[1]	2,564	1,789
Income taxes receivable	[1]	(87)	(241)
Total current assets	[1]	133,780	178,725
Property and equipment - net	[1]	314,264	250,770
Restricted cash and investments	[1]	15
Other assets	[1]	1,153	203
Total assets	[1]	449,212	429,698
Current Liabilities:
Accounts payable	[1]	3,611	5,861
Accrued and other current liabilities	[1]	10,284	15,621
Total current liabilities	[1]	13,895	21,482
Long-term debt:
Deferred income taxes and other long-term liabilities	[1]	83,877	77,368
Shareholders' Equity:
Common stock and paid-in capital	[1]	79,191	79,061
Retained earnings	[1]	272,249	251,787
Total shareholders' equity	[1]	351,440	330,848
Total liabilities and shareholders' equity	[1]	449,212	429,698
Eliminations [Member]
Current Assets:
Intercompany receivable		(41,796)	(90,943)
Total current assets		(41,796)	(90,943)
Investment in subsidiaries		(351,440)	(330,848)
Total assets		(393,236)	(421,791)
Current Liabilities:
Intercompany payable		(41,796)	(90,943)
Total current liabilities		(41,796)	(90,943)
Shareholders' Equity:
Common stock and paid-in capital		(79,191)	(79,061)
Retained earnings		(272,249)	(251,787)
Total shareholders' equity		(351,440)	(330,848)
Total liabilities and shareholders' equity		$ (393,236)	$ (421,791)

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