Condensed Consolidating Financial Information - Guarantor Subsidiaries - Condensed Consolidating Balance Sheets (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017	Dec. 31, 2016
Current Assets:
Cash	$ 5,852	$ 8,770	$ 2,323	$ 2,596
Short-term investments	60,317	64,237
Accounts receivable - net	194,332	185,979
Inventories of spare parts – net	77,747	80,881
Prepaid expenses	13,150	11,475
Income taxes receivable	623	1,271
Total current assets	352,021	352,613
Investment in subsidiaries and others		0
Property and equipment – net	933,583	946,765
Restricted cash and investments	12,076	12,396
Other assets	9,605	8,741
Deferred income taxes	3,237	3,309
Goodwill	61,299	61,299
Intangibles	15,946	16,723
Total assets	1,387,767	1,401,846
Current Liabilities:
Short-term debt	620,991
Accounts payable	50,645	37,186
Accrued and other current liabilities	40,626	41,850
Total current liabilities	712,262	79,036
Long-term debt		615,994
Deferred income taxes and other long-term liabilities	83,940	94,162
Shareholders’ Equity:
Common stock and paid-in capital	312,015	309,933
Accumulated other comprehensive income (loss)	624	(280)
Retained earnings	278,926	303,001
Total shareholders’ equity	591,565	612,654	584,061	599,778
Total liabilities and shareholders’ equity	1,387,767	1,401,846
Eliminations [Member]
Current Assets:
Accounts receivable - net	(2,187)	(17,004)
Intercompany receivable	(119,963)	(126,366)
Total current assets	(122,150)	(143,370)
Investment in subsidiaries and others	(405,560)	(397,301)
Total assets	(527,710)	(540,671)
Current Liabilities:
Short-term debt	0
Accounts payable	(2,261)	(17,005)
Accrued and other current liabilities		0
Intercompany payable	(119,889)	(126,365)
Total current liabilities	(122,150)	(143,370)
Shareholders’ Equity:
Common stock and paid-in capital	(79,462)	(79,326)
Retained earnings	(326,098)	(317,975)
Total shareholders’ equity	(405,560)	(397,301)
Total liabilities and shareholders’ equity	(527,710)	(540,671)
Parent Company Only (issuer) [Member]
Current Assets:
Cash	53	47	48	36
Short-term investments	60,317	64,237
Accounts receivable - net	76,291	90,077
Inventories of spare parts – net	65,522	68,737
Prepaid expenses	9,494	8,348
Income taxes receivable	111	345
Total current assets	211,788	231,791
Investment in subsidiaries and others	405,560	397,301
Property and equipment – net	603,926	617,488
Restricted cash and investments	12,076	12,382
Other assets	140,158	139,754
Total assets	1,373,508	1,398,716
Current Liabilities:
Short-term debt	620,991
Accounts payable	41,401	28,130
Accrued and other current liabilities	23,230	23,147
Intercompany payable	100,982	113,387
Total current liabilities	786,604	164,664
Long-term debt		615,994
Deferred income taxes and other long-term liabilities	(4,034)	5,404
Shareholders’ Equity:
Common stock and paid-in capital	312,015	309,933
Accumulated other comprehensive income (loss)	(3)	(280)
Retained earnings	278,926	303,001
Total shareholders’ equity	590,938	612,654
Total liabilities and shareholders’ equity	1,373,508	1,398,716
Guarantor Subsidiaries [Member]
Current Assets:
Cash	1,305	1,072	1,328	2,100
Accounts receivable - net	84,882	74,886
Intercompany receivable	119,963	126,366
Inventories of spare parts – net	9,512	9,049
Prepaid expenses	2,715	1,898
Income taxes receivable	512	9
Total current assets	218,889	213,280
Property and equipment – net	286,820	284,984
Other assets	794	908
Total assets	506,503	499,172
Current Liabilities:
Short-term debt	0
Accounts payable	4,039	4,636
Accrued and other current liabilities	10,891	10,577
Total current liabilities	14,930	15,213
Deferred income taxes and other long-term liabilities	84,539	84,300
Shareholders’ Equity:
Common stock and paid-in capital	77,951	77,951
Retained earnings	329,083	321,708
Total shareholders’ equity	407,034	399,659
Total liabilities and shareholders’ equity	506,503	499,172
Non-Guarantor Subsidiaries [Member]
Current Assets:
Cash	4,494	7,651	$ 947	$ 460
Accounts receivable - net	35,346	38,020
Inventories of spare parts – net	2,713	3,095
Prepaid expenses	941	1,229
Income taxes receivable	0	917
Total current assets	43,494	50,912
Property and equipment – net	42,837	44,293
Restricted cash and investments	0	14
Other assets	(131,347)	(131,921)
Deferred income taxes	3,237	3,309
Goodwill	61,299	61,299
Intangibles	15,946	16,723
Total assets	35,466	44,629
Current Liabilities:
Short-term debt	0
Accounts payable	7,466	21,425
Accrued and other current liabilities	6,505	8,126
Intercompany payable	18,907	12,978
Total current liabilities	32,878	42,529
Deferred income taxes and other long-term liabilities	3,435	4,458
Shareholders’ Equity:
Common stock and paid-in capital	1,511	1,375
Accumulated other comprehensive income (loss)	627
Retained earnings	(2,985)	(3,733)
Total shareholders’ equity	(847)	(2,358)
Total liabilities and shareholders’ equity	$ 35,466	$ 44,629