Condensed Consolidating Financial Information - Guarantor Subsidiaries - Condensed Consolidating Balance Sheets (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017
Current Assets:
Cash and cash equivalents	$ 15,884	$ 50,874	$ 7,431	$ 8,770
Short-term investments	73,336	14,232
Accounts receivable - net	178,813	182,465
Inventories of spare parts – net	52,988	56,698
Prepaid expenses	13,944	11,419
Income taxes receivable	942	950
Total current assets	335,907	316,638
Investment in subsidiaries and others		0
Property and equipment – net	896,436	902,485
Right of use assets	144,198
Restricted cash and investments	19,789	19,781
Other assets	18,981	18,378
Deferred income tax	4,521	4,944
Goodwill		0
Intangibles		0
Total assets	1,419,832	1,262,226
Current Liabilities:
Senior Notes issued March 17, 2014, net of debt issuance cost of $260	0	499,740
Accounts payable	38,854	50,289
Accrued and other current liabilities	32,083
Accrued and other current liabilities	31,208	45,292
Current portion of operating lease liabilities	26,544
Total current liabilities	97,481	595,321
Long-term debt	192,860	129,235
Deferred income taxes and other long-term liabilities	58,422	64,237
Long-term operating lease liabilities	118,436
Liabilities subject to compromise	513,125
Shareholders’ Equity:
Common stock and paid-in capital	316,400	315,898
Accumulated other comprehensive income (loss)	(3,138)	(3,952)
Retained earnings	126,246	161,487
Total shareholders’ equity	439,508	473,433	$ 597,735	$ 612,654
Total liabilities and shareholders’ equity	1,419,832	1,262,226
Eliminations [Member]
Current Assets:
Accounts receivable - net	(4,756)	(3,823)
Intercompany receivable	(120,874)	(91,468)
Total current assets	(125,630)	(95,291)
Investment in subsidiaries and others	(470,310)	(471,790)
Total assets	(595,940)	(567,081)
Current Liabilities:
Accounts payable	(4,756)	(3,839)
Accrued and other current liabilities	0
Accrued and other current liabilities		0
Intercompany payable	(120,874)	(91,452)
Total current liabilities	(125,630)	(95,291)
Shareholders’ Equity:
Common stock and paid-in capital	(210,248)	(210,601)
Retained earnings	(260,062)	(261,189)
Total shareholders’ equity	(470,310)	(471,790)
Total liabilities and shareholders’ equity	(595,940)	(567,081)
Parent Company Only (issuer) [Member] | Reportable Legal Entities [Member]
Current Assets:
Cash and cash equivalents	1,335	39,091
Short-term investments	73,336	14,232
Accounts receivable - net	61,431	64,416
Inventories of spare parts – net	39,525	43,933
Prepaid expenses	9,605	7,295
Income taxes receivable	385	394
Total current assets	185,617	169,361
Investment in subsidiaries and others	470,310	471,790
Property and equipment – net	567,578	576,763
Right of use assets	122,365
Restricted cash and investments	19,789	19,781
Other assets	17,207	17,179
Total assets	1,382,866	1,254,874
Current Liabilities:
Senior Notes issued March 17, 2014, net of debt issuance cost of $260		499,740
Accounts payable	31,793	44,089
Accrued and other current liabilities	14,000
Accrued and other current liabilities		25,158
Current portion of operating lease liabilities	21,043
Intercompany payable	64,894	74,336
Total current liabilities	131,730	643,323
Long-term debt	192,860	129,235
Deferred income taxes and other long-term liabilities	475	4,934
Long-term operating lease liabilities	102,033
Liabilities subject to compromise	513,125
Shareholders’ Equity:
Common stock and paid-in capital	316,400	315,898
Accumulated other comprehensive income (loss)	(3)	(3)
Retained earnings	126,246	161,487
Total shareholders’ equity	442,643	477,382
Total liabilities and shareholders’ equity	1,382,866	1,254,874
Guarantor Subsidiaries [Member] | Reportable Legal Entities [Member]
Current Assets:
Cash and cash equivalents	1,584	1,259
Accounts receivable - net	94,491	93,060
Intercompany receivable	98,314	91,468
Inventories of spare parts – net	10,438	9,577
Prepaid expenses	2,881	2,520
Income taxes receivable	557	556
Total current assets	208,265	198,440
Property and equipment – net	289,267	287,375
Right of use assets	12,130
Other assets	1,150	1,199
Total assets	510,812	487,014
Current Liabilities:
Accounts payable	6,065	4,587
Accrued and other current liabilities	12,183
Accrued and other current liabilities		12,335
Current portion of operating lease liabilities	3,406
Intercompany payable	13,452
Total current liabilities	35,106	16,922
Deferred income taxes and other long-term liabilities	57,787	58,752
Long-term operating lease liabilities	8,809
Shareholders’ Equity:
Common stock and paid-in capital	77,951	77,951
Retained earnings	331,159	333,389
Total shareholders’ equity	409,110	411,340
Total liabilities and shareholders’ equity	510,812	487,014
Non-Guarantor Subsidiaries [Member] | Reportable Legal Entities [Member]
Current Assets:
Cash and cash equivalents	12,965	10,524
Accounts receivable - net	27,647	28,812
Intercompany receivable	22,560
Inventories of spare parts – net	3,025	3,188
Prepaid expenses	1,458	1,604
Income taxes receivable	0	0
Total current assets	67,655	44,128
Property and equipment – net	39,591	38,347
Right of use assets	9,703
Restricted cash and investments		0
Other assets	624	0
Deferred income tax	4,521	4,944
Goodwill		0
Intangibles		0
Total assets	122,094	87,419
Current Liabilities:
Accounts payable	5,752	5,452
Accrued and other current liabilities	5,900
Accrued and other current liabilities		7,799
Current portion of operating lease liabilities	2,095
Intercompany payable	42,528	17,116
Total current liabilities	56,275	30,367
Deferred income taxes and other long-term liabilities	160	551
Long-term operating lease liabilities	7,594
Shareholders’ Equity:
Common stock and paid-in capital	132,297	132,650
Accumulated other comprehensive income (loss)	(3,135)	(3,949)
Retained earnings	(71,097)	(72,200)
Total shareholders’ equity	58,065	56,501
Total liabilities and shareholders’ equity	$ 122,094	$ 87,419