Condensed Consolidating Financial Information - Guarantor Subsidiaries - Condensed Consolidating Balance Sheets (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017
Current Assets:
Cash and cash equivalents	$ 21,882		$ 50,874	$ 5,852		$ 8,770
Short-term investments	76,141		14,232
Accounts receivable – net	183,344		182,465
Inventories of spare parts – net	54,242		56,698
Prepaid expenses	13,072		11,419
Income taxes receivable	509		950
Total current assets	349,190		316,638
Property and equipment – net	885,669		902,485
Right of use assets	144,944
Restricted cash and investments	19,739		19,781
Other assets	20,937		18,378
Deferred income taxes	5,137		4,944
Total assets	1,425,616		1,262,226
Current Liabilities:
Senior Notes issued March 17, 2014			499,740
Accounts payable	70,615		50,289
Accrued and other current liabilities	38,483		45,292
Current portion of operating lease liabilities	27,535
Current maturities of long term debt	1,750		500,000
Total current liabilities	138,383		595,321
Long-term Debt	192,450		129,235
Deferred income taxes and other long-term liabilities	173,804		64,237
Liabilities subject to compromise	513,125
Shareholders’ Equity:
Common stock and paid-in capital	317,725		315,898
Accumulated other comprehensive income (loss)	(4,040)		(3,952)
Retained earnings	94,169		161,487
Total shareholders’ equity	407,854	$ 439,508	473,433	$ 591,565	$ 597,735	$ 612,654
Total liabilities and shareholders’ equity	1,425,616		1,262,226
Eliminations [Member]
Current Assets:
Accounts receivable – net	(6,180)		(3,823)
Intercompany receivable	(120,732)		(91,468)
Total current assets	(126,912)		(95,291)
Investment in subsidiaries	(475,391)		(471,790)
Total assets	(602,303)		(567,081)
Current Liabilities:
Accounts payable	(6,180)		(3,839)
Intercompany payable	(120,732)		(91,452)
Total current liabilities	(126,912)		(95,291)
Shareholders’ Equity:
Common stock and paid-in capital	(210,377)		(210,601)
Retained earnings	(265,014)		(261,189)
Total shareholders’ equity	(475,391)		(471,790)
Total liabilities and shareholders’ equity	(602,303)		(567,081)
Parent Company Only (issuer) [Member] | Reportable Legal Entities [Member]
Current Assets:
Cash and cash equivalents	8,170		39,091
Short-term investments	76,141		14,232
Accounts receivable – net	64,467		64,416
Inventories of spare parts – net	29,871		43,933
Prepaid expenses	8,944		7,295
Income taxes receivable	7		394
Total current assets	187,600		169,361
Investment in subsidiaries	475,391		471,790
Property and equipment – net	562,880		576,763
Right of use assets	116,918
Restricted cash and investments	19,739		19,781
Other assets	18,678		17,179
Total assets	1,381,206		1,254,874
Current Liabilities:
Senior Notes issued March 17, 2014			499,740
Accounts payable	64,526		44,089
Accrued and other current liabilities	19,667		25,158
Current portion of operating lease liabilities	20,823
Current maturities of long term debt	1,750
Intercompany payable	63,263		74,336
Total current liabilities	170,029		643,323
Long-term Debt	192,450		129,235
Deferred income taxes and other long-term liabilities	93,711		4,934
Liabilities subject to compromise	513,125
Shareholders’ Equity:
Common stock and paid-in capital	317,725		315,898
Accumulated other comprehensive income (loss)	(3)		(3)
Retained earnings	94,169		161,487
Total shareholders’ equity	411,891		477,382
Total liabilities and shareholders’ equity	1,381,206		1,254,874
Guarantor Subsidiaries [Member] | Reportable Legal Entities [Member]
Current Assets:
Cash and cash equivalents	2,223		1,259
Accounts receivable – net	101,381		93,060
Intercompany receivable	98,616		91,468
Inventories of spare parts – net	20,929		9,577
Prepaid expenses	2,805		2,520
Income taxes receivable	502		556
Total current assets	226,456		198,440
Property and equipment – net	283,231		287,375
Right of use assets	11,652
Other assets	1,071		1,199
Total assets	522,410		487,014
Current Liabilities:
Accounts payable	6,437		4,587
Accrued and other current liabilities	13,500		12,335
Current portion of operating lease liabilities	3,404
Intercompany payable	17,125
Total current liabilities	40,466		16,922
Deferred income taxes and other long-term liabilities	67,234		58,752
Shareholders’ Equity:
Common stock and paid-in capital	77,951		77,951
Retained earnings	336,759		333,389
Total shareholders’ equity	414,710		411,340
Total liabilities and shareholders’ equity	522,410		487,014
Non-Guarantor Subsidiaries [Member] | Reportable Legal Entities [Member]
Current Assets:
Cash and cash equivalents	11,489		10,524
Accounts receivable – net	23,676		28,812
Intercompany receivable	22,116
Inventories of spare parts – net	3,442		3,188
Prepaid expenses	1,323		1,604
Total current assets	62,046		44,128
Property and equipment – net	39,558		38,347
Right of use assets	16,374
Other assets	1,188
Deferred income taxes	5,137		4,944
Total assets	124,303		87,419
Current Liabilities:
Accounts payable	5,832		5,452
Accrued and other current liabilities	5,316		7,799
Current portion of operating lease liabilities	3,308
Intercompany payable	40,344		17,116
Total current liabilities	54,800		30,367
Deferred income taxes and other long-term liabilities	12,859		551
Shareholders’ Equity:
Common stock and paid-in capital	132,426		132,650
Accumulated other comprehensive income (loss)	(4,037)		(3,949)
Retained earnings	(71,745)		(72,200)
Total shareholders’ equity	56,644		56,501
Total liabilities and shareholders’ equity	$ 124,303		$ 87,419