Condensed Consolidating Financial Information - Guarantor Subsidiaries - Condensed Consolidating Balance Sheets (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
Current Assets:
Cash	$ 16,453	$ 8,770	$ 2,991	$ 2,596
Short-term investments	30,509	64,237
Accounts receivable - net	197,407	185,979
Inventories of spare parts – net	74,352	80,881
Prepaid expenses	13,201	11,475
Income taxes receivable	649	1,271
Total current assets	332,571	352,613
Investment in subsidiaries and others		0
Property and equipment – net	923,577	946,765
Restricted investments	19,781	12,396
Other assets	10,142	8,741
Deferred income taxes	1,009	3,309
Goodwill	61,299	61,299
Intangibles	15,557	16,723
Total assets	1,363,936	1,401,846
Current Liabilities:
Short-term debt	499,429
Accounts payable	39,209	37,186
Accrued and other current liabilities	39,557	41,850
Total current liabilities	578,195	79,036
Long-term debt	129,126	615,994
Deferred income taxes and other long-term liabilities	79,230	94,162
Shareholders’ Equity:
Common stock and paid-in capital	314,085	309,933
Accumulated other comprehensive income (loss)	(4,094)	(280)
Retained earnings	267,394	303,001
Total shareholders’ equity	577,385	612,654	582,061	599,778
Total liabilities and shareholders’ equity	1,363,936	1,401,846
Eliminations [Member]
Current Assets:
Accounts receivable - net	(3,477)	(17,004)
Intercompany receivable	(92,338)	(126,366)
Total current assets	(95,815)	(143,370)
Investment in subsidiaries and others	(415,980)	(397,301)
Total assets	(511,795)	(540,671)
Current Liabilities:
Short-term debt	0
Accounts payable	(3,477)	(17,005)
Accrued and other current liabilities		0
Intercompany payable	(92,338)	(126,365)
Total current liabilities	(95,815)	(143,370)
Shareholders’ Equity:
Common stock and paid-in capital	(79,462)	(79,326)
Retained earnings	(336,518)	(317,975)
Total shareholders’ equity	(415,980)	(397,301)
Total liabilities and shareholders’ equity	(511,795)	(540,671)
Parent Company Only (issuer) [Member]
Current Assets:
Cash	9,905	47	50	36
Short-term investments	30,509	64,237
Accounts receivable - net	75,970	90,077
Inventories of spare parts – net	60,914	68,737
Prepaid expenses	9,364	8,348
Income taxes receivable	139	345
Total current assets	186,801	231,791
Investment in subsidiaries and others	415,980	397,301
Property and equipment – net	598,363	617,488
Restricted investments	19,781	12,382
Other assets	140,265	139,754
Total assets	1,361,190	1,398,716
Current Liabilities:
Short-term debt	499,429
Accounts payable	32,458	28,130
Accrued and other current liabilities	18,781	23,147
Intercompany payable	81,384	113,387
Total current liabilities	632,052	164,664
Long-term debt	129,126	615,994
Deferred income taxes and other long-term liabilities	18,536	5,404
Shareholders’ Equity:
Common stock and paid-in capital	314,085	309,933
Accumulated other comprehensive income (loss)	(3)	(280)
Retained earnings	267,394	303,001
Total shareholders’ equity	581,476	612,654
Total liabilities and shareholders’ equity	1,361,190	1,398,716
Guarantor Subsidiaries [Member]
Current Assets:
Cash	1,880	1,072	1,071	2,100
Accounts receivable - net	95,340	74,886
Intercompany receivable	92,338	126,366
Inventories of spare parts – net	10,646	9,049
Prepaid expenses	2,640	1,898
Income taxes receivable	510	9
Total current assets	203,354	213,280
Property and equipment – net	286,015	284,984
Other assets	1,042	908
Total assets	490,411	499,172
Current Liabilities:
Short-term debt	0
Accounts payable	5,200	4,636
Accrued and other current liabilities	14,053	10,577
Total current liabilities	19,253	15,213
Deferred income taxes and other long-term liabilities	57,439	84,300
Shareholders’ Equity:
Common stock and paid-in capital	77,952	77,951
Retained earnings	335,767	321,708
Total shareholders’ equity	413,719	399,659
Total liabilities and shareholders’ equity	490,411	499,172
Non-Guarantor Subsidiaries [Member]
Current Assets:
Cash	4,668	7,651	$ 1,870	$ 460
Accounts receivable - net	29,574	38,020
Inventories of spare parts – net	2,792	3,095
Prepaid expenses	1,197	1,229
Income taxes receivable	0	917
Total current assets	38,231	50,912
Property and equipment – net	39,199	44,293
Restricted investments	0	14
Other assets	(131,165)	(131,921)
Deferred income taxes	1,009	3,309
Goodwill	61,299	61,299
Intangibles	15,557	16,723
Total assets	24,130	44,629
Current Liabilities:
Short-term debt	0
Accounts payable	5,028	21,425
Accrued and other current liabilities	6,723	8,126
Intercompany payable	10,954	12,978
Total current liabilities	22,705	42,529
Deferred income taxes and other long-term liabilities	3,255	4,458
Shareholders’ Equity:
Common stock and paid-in capital	1,510	1,375
Accumulated other comprehensive income (loss)	(4,091)
Retained earnings	751	(3,733)
Total shareholders’ equity	(1,830)	(2,358)
Total liabilities and shareholders’ equity	$ 24,130	$ 44,629