CONDENSED CONSOLIDATING FINANCIAL INFORMATION - Condensed Consolidating Balance Sheets (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012	Jun. 30, 2012	Dec. 31, 2011
Current Assets:
Cash	$ 12,190	$ 2,849	$ 1,424	$ 5,091
Short-term investments	76,214	50,601
Accounts receivable - net	155,709	137,179
Inventories of spare parts - net	67,403	66,074
Prepaid expenses	11,493	10,137
Work in progress	77,921	77,764
Other current assets		988
Deferred income taxes	11,967	11,967
Income taxes receivable	1,623	1,613
Total current assets	416,389	363,146
Property and equipment - net	756,928	749,501
Restricted investments	14,685	14,685
Other assets	15,853	20,562
Total assets	1,203,855	1,147,894
Current Liabilities:
Accounts payable	40,754	26,308
Accrued and other current liabilities	122,111	117,162
Total current liabilities	162,865	143,470
Long-term debt	375,723	386,755
Deferred income taxes and other long-term liabilities	137,364	118,054
Shareholders' Equity:
Common stock and paid-in capital	297,076	297,113
Accumulated other comprehensive loss	(80)	(51)
Retained earnings	230,907	202,553
Total shareholders' equity	527,903	499,615	485,624	477,337
Total liabilities and shareholders' equity	1,203,855	1,147,894
Parent Company Only (issuer) [Member]
Current Assets:
Cash	1,157	552	52	4,313
Short-term investments	76,214	50,601
Accounts receivable - net	89,105	80,148
Intercompany receivable	126,255	115,300
Inventories of spare parts - net	67,277	65,951
Prepaid expenses	9,244	8,354
Work in progress	77,921	77,764
Other current assets		988
Deferred income taxes	11,966	11,967
Income taxes receivable	1,393	1,395
Total current assets	460,532	413,020
Investment in subsidiaries	108,241	96,706
Property and equipment - net	556,665	559,686
Restricted investments	14,685	14,685
Other assets	15,503	19,726
Total assets	1,155,626	1,103,823
Current Liabilities:
Accounts payable	33,519	21,188
Accrued and other current liabilities	109,387	105,875
Total current liabilities	142,906	127,063
Long-term debt	375,723	386,755
Deferred income taxes and other long-term liabilities	109,094	90,390
Shareholders' Equity:
Common stock and paid-in capital	297,076	297,113
Accumulated other comprehensive loss	(80)	(51)
Retained earnings	230,907	202,553
Total shareholders' equity	527,903	499,615
Total liabilities and shareholders' equity	1,155,626	1,103,823
Guarantor Subsidiaries [Member]
Current Assets:
Cash	11,033	2,297	1,372	778
Accounts receivable - net	68,473	61,005
Inventories of spare parts - net	126	123
Prepaid expenses	2,249	1,783
Income taxes receivable	231	218
Total current assets	82,112	65,426
Property and equipment - net	200,263	189,815
Other assets	350	836
Total assets	282,725	256,077
Current Liabilities:
Accounts payable	7,235	5,120
Accrued and other current liabilities	12,724	11,287
Intercompany payable	126,255	115,300
Total current liabilities	146,214	131,707
Deferred income taxes and other long-term liabilities	28,270	27,664
Shareholders' Equity:
Common stock and paid-in capital	2,674	2,674
Retained earnings	105,567	94,032
Total shareholders' equity	108,241	96,706
Total liabilities and shareholders' equity	282,725	256,077
Eliminations [Member]
Current Assets:
Cash
Intercompany receivable	(126,255)	(115,300)
Total current assets	(126,255)	(115,300)
Investment in subsidiaries	(108,241)	(96,706)
Total assets	(234,496)	(212,006)
Current Liabilities:
Intercompany payable	(126,255)	(115,300)
Total current liabilities	(126,255)	(115,300)
Shareholders' Equity:
Common stock and paid-in capital	(2,674)	(2,674)
Retained earnings	(105,567)	(94,032)
Total shareholders' equity	(108,241)	(96,706)
Total liabilities and shareholders' equity	$ (234,496)	$ (212,006)