Condensed Consolidating Financial Information - Guarantor Subsidiaries - Condensed Consolidating Balance Sheets (Detail) - USD ($) $ in Thousands		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Current Assets:
Cash		$ 2,596	$ 2,407	$ 6,270	$ 934
Short-term investments		289,806	284,523
Accounts receivable - net		138,265	144,778
Inventories of spare parts - net		70,402	69,491
Prepaid expenses		9,259	8,951
Deferred income taxes		10,798	10,379
Income taxes receivable		540	761
Total current assets		521,666	521,290
Property and equipment - net		903,977	883,529
Restricted cash and investments		13,038	15,336
Other assets		9,759	6,178
Total assets		1,448,440	1,426,333
Current Liabilities:
Accounts payable		28,704	31,373
Accrued and other current liabilities		28,346	44,759
Total current liabilities		57,050	76,132
Long-term debt		631,247	553,501
Deferred income taxes and other long-term liabilities		160,365	169,702
Shareholders' Equity:
Common stock and paid-in capital		305,815	306,444
Accumulated other comprehensive income (loss)		(478)	(567)
Retained earnings		294,441	321,121
Total shareholders' equity		599,778	626,998	597,068	559,965
Total liabilities and shareholders' equity		1,448,440	1,426,333
Parent Company Only (issuer) [Member]
Current Assets:
Cash		36	46	51	52
Short-term investments		289,806	284,523
Accounts receivable - net		71,458	70,336
Inventories of spare parts - net		61,834	60,060
Prepaid expenses		6,990	7,162
Deferred income taxes		10,798	10,379
Income taxes receivable		558	1,002
Total current assets		441,480	433,508
Investment in subsidiaries and others		353,160	330,848
Property and equipment - net		589,104	632,759
Restricted cash and investments		13,023	15,336
Other assets		8,660	5,975
Total assets		1,405,427	1,418,426
Current Liabilities:
Accounts payable		22,744	25,512
Accrued and other current liabilities		18,725	29,138
Intercompany payable		57,904	90,943
Total current liabilities		99,373	145,593
Long-term debt		631,247	553,501
Deferred income taxes and other long-term liabilities		75,029	92,334
Shareholders' Equity:
Common stock and paid-in capital		305,815	306,444
Accumulated other comprehensive income (loss)		(478)	(567)
Retained earnings		294,441	321,121
Total shareholders' equity		599,778	626,998
Total liabilities and shareholders' equity		1,405,427	1,418,426
Guarantor Subsidiaries [Member]
Current Assets:
Cash	[1]	2,560	2,361	$ 6,219	$ 882
Accounts receivable - net	[1]	66,807	74,442
Intercompany receivable	[1]	57,904	90,943
Inventories of spare parts - net	[1]	8,568	9,431
Prepaid expenses	[1]	2,269	1,789
Income taxes receivable	[1]	(18)	(241)
Total current assets	[1]	138,090	178,725
Property and equipment - net	[1]	314,873	250,770
Restricted cash and investments	[1]	15
Other assets	[1]	1,099	203
Total assets	[1]	454,077	429,698
Current Liabilities:
Accounts payable	[1]	5,960	5,861
Accrued and other current liabilities	[1]	9,621	15,621
Total current liabilities	[1]	15,581	21,482
Deferred income taxes and other long-term liabilities	[1]	85,336	77,368
Shareholders' Equity:
Common stock and paid-in capital	[1]	79,191	79,061
Retained earnings	[1]	273,969	251,787
Total shareholders' equity	[1]	353,160	330,848
Total liabilities and shareholders' equity	[1]	454,077	429,698
Eliminations [Member]
Current Assets:
Intercompany receivable		(57,904)	(90,943)
Total current assets		(57,904)	(90,943)
Investment in subsidiaries and others		(353,160)	(330,848)
Total assets		(411,064)	(421,791)
Current Liabilities:
Intercompany payable		(57,904)	(90,943)
Total current liabilities		(57,904)	(90,943)
Shareholders' Equity:
Common stock and paid-in capital		(79,191)	(79,061)
Retained earnings		(273,969)	(251,787)
Total shareholders' equity		(353,160)	(330,848)
Total liabilities and shareholders' equity		$ (411,064)	$ (421,791)

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