Condensed Consolidating Financial Information - Guarantor Subsidiaries - Condensed Consolidating Balance Sheets (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016	Dec. 31, 2015
Current Assets:
Cash	$ 2,991	$ 2,596	$ 2,639	$ 2,407
Short-term investments	204,036	289,806
Accounts receivable - net	152,737	138,265
Inventories of spare parts – net	79,245	70,402
Prepaid expenses	12,006	9,259
Deferred income taxes	10,798	10,798
Income taxes receivable	509	540
Total current assets	462,322	521,666
Property and equipment – net	910,327	903,977
Restricted cash and investments	12,396	13,038
Other assets	9,032	9,759
Total assets	1,394,077	1,448,440
Current Liabilities:
Accounts payable	30,607	28,704
Accrued and other current liabilities	33,703	28,346
Total current liabilities	64,310	57,050
Long-term debt	598,182	631,247
Deferred income taxes and other long-term liabilities	149,524	160,365
Shareholders’ Equity:
Common stock and paid-in capital	308,634	305,815
Accumulated other comprehensive loss	(244)	(478)
Retained earnings	273,671	294,441
Total shareholders’ equity	582,061	599,778	621,487	626,998
Total liabilities and shareholders’ equity	1,394,077	1,448,440
Eliminations [Member]
Current Assets:
Accounts receivable - net	(10,772)
Intercompany receivable	(120,376)	(57,904)
Total current assets	(131,148)	(57,904)
Investment in subsidiaries and others	(390,840)	(353,160)
Total assets	(521,988)	(411,064)
Current Liabilities:
Accounts payable	(10,772)
Intercompany payable	(120,376)	(57,904)
Total current liabilities	(131,148)	(57,904)
Shareholders’ Equity:
Common stock and paid-in capital	(79,326)	(79,191)
Retained earnings	(311,514)	(273,969)
Total shareholders’ equity	(390,840)	(353,160)
Total liabilities and shareholders’ equity	(521,988)	(411,064)
Parent Company Only (issuer) [Member]
Current Assets:
Cash	50	36	36	46
Short-term investments	204,036	289,806
Accounts receivable - net	77,061	71,458
Inventories of spare parts – net	70,617	61,834
Prepaid expenses	9,916	6,990
Deferred income taxes	10,798	10,798
Income taxes receivable	500	558
Total current assets	372,978	441,480
Investment in subsidiaries and others	390,840	353,160
Property and equipment – net	621,885	589,104
Restricted cash and investments	12,381	13,023
Other assets	8,076	8,660
Total assets	1,406,160	1,405,427
Current Liabilities:
Accounts payable	26,587	22,744
Accrued and other current liabilities	22,198	18,725
Intercompany payable	112,015	57,904
Total current liabilities	160,800	99,373
Long-term debt	598,182	631,247
Deferred income taxes and other long-term liabilities	65,117	75,029
Shareholders’ Equity:
Common stock and paid-in capital	308,634	305,815
Accumulated other comprehensive loss	(244)	(478)
Retained earnings	273,671	294,441
Total shareholders’ equity	582,061	599,778
Total liabilities and shareholders’ equity	1,406,160	1,405,427
Guarantor Subsidiaries [Member]
Current Assets:
Cash	1,071	2,100	$ 2,603	$ 2,361
Accounts receivable - net	71,727	66,807
Intercompany receivable	120,376	57,904
Inventories of spare parts – net	8,628	8,568
Prepaid expenses	1,948	2,269
Income taxes receivable	9	(18)
Total current assets	203,759	138,090
Property and equipment – net	287,841	314,873
Restricted cash and investments	0	15
Other assets	956	1,099
Total assets	492,556	454,077
Current Liabilities:
Accounts payable	3,557	5,960
Accrued and other current liabilities	9,961	9,621
Total current liabilities	13,518	15,581
Deferred income taxes and other long-term liabilities	83,891	85,336
Shareholders’ Equity:
Common stock and paid-in capital	77,951	79,191
Retained earnings	317,196	273,969
Total shareholders’ equity	395,147	353,160
Total liabilities and shareholders’ equity	492,556	454,077
Non-Guarantor Subsidiaries [Member]
Current Assets:
Cash	1,870	460
Accounts receivable - net	14,721
Prepaid expenses	142
Income taxes receivable	0
Total current assets	16,733
Property and equipment – net	601
Restricted cash and investments	15
Total assets	17,349
Current Liabilities:
Accounts payable	11,235
Accrued and other current liabilities	1,544
Intercompany payable	8,361
Total current liabilities	21,140
Deferred income taxes and other long-term liabilities	516
Shareholders’ Equity:
Common stock and paid-in capital	1,375
Retained earnings	(5,682)
Total shareholders’ equity	(4,307)
Total liabilities and shareholders’ equity	$ 17,349
Guarantor and Non-Guarantor Subsidiaries [Member]
Current Assets:
Cash		$ 2,560