Condensed Consolidating Financial Information (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2012	Dec. 31, 2011
Current Assets:
Cash	$ 3,577	$ 2,849	$ 1,047	$ 5,091
Short-term investments	46,020	50,601
Accounts receivable - net	138,119	141,153
Inventories of spare parts - net	66,273	66,074
Prepaid expenses	11,282	10,137
Work in progress	77,921	77,764
Other current assets	996	988
Deferred income taxes	11,966	11,967
Income taxes receivable	1,599	1,613
Total current assets	357,753	363,146
Property and equipment - net	764,373	749,501
Restricted investments	14,685	14,685
Other assets	20,110	20,562
Total assets	1,156,921	1,147,894
Current Liabilities:
Accounts payable	26,350	26,308
Accrued and other current liabilities	120,163	117,162
Total current liabilities	146,513	143,470
Long-term debt	379,155	386,755
Deferred income taxes and other long-term liabilities	123,421	118,054
Shareholders' Equity:
Common stock and paid-in capital	296,477	297,113
Accumulated other comprehensive loss	(36)	(51)
Retained earnings	211,391	202,553
Total shareholders' equity	507,832	499,615	479,561	477,337
Total liabilities and shareholders' equity	1,156,921	1,147,894
Parent Company Only (issuer) [Member]
Current Assets:
Cash	2,259	552	53	4,313
Short-term investments	46,020	50,601
Accounts receivable - net	75,519	80,148
Intercompany receivable	116,597	115,300
Inventories of spare parts - net	66,150	65,951
Prepaid expenses	9,774	8,354
Work in progress	77,921	77,764
Other current assets	996	988
Deferred income taxes	11,966	11,967
Income taxes receivable	1,381	1,395
Total current assets	408,583	413,020
Investment in subsidiaries	102,124	96,706
Property and equipment - net	570,787	559,686
Restricted investments	14,685	14,685
Other assets	19,494	19,726
Total assets	1,115,673	1,103,823
Current Liabilities:
Accounts payable	21,688	21,188
Accrued and other current liabilities	111,397	105,875
Total current liabilities	133,085	127,063
Long-term debt	379,155	386,755
Deferred income taxes and other long-term liabilities	95,601	90,390
Shareholders' Equity:
Common stock and paid-in capital	296,477	297,113
Accumulated other comprehensive loss	(36)	(51)
Retained earnings	211,391	202,553
Total shareholders' equity	507,832	499,615
Total liabilities and shareholders' equity	1,115,673	1,103,823
Guarantor Subsidiaries [Member]
Current Assets:
Cash	1,318	2,297	994	778
Accounts receivable - net	62,600	61,005
Inventories of spare parts - net	123	123
Prepaid expenses	1,508	1,783
Income taxes receivable	218	218
Total current assets	65,767	65,426
Property and equipment - net	193,586	189,815
Other assets	616	836
Total assets	259,969	256,077
Current Liabilities:
Accounts payable	4,662	5,120
Accrued and other current liabilities	8,766	11,287
Intercompany payable	116,597	115,300
Total current liabilities	130,025	131,707
Deferred income taxes and other long-term liabilities	27,820	27,664
Shareholders' Equity:
Common stock and paid-in capital		2,674
Accumulated other comprehensive loss	2,674
Retained earnings	99,450	94,032
Total shareholders' equity	102,124	96,706
Total liabilities and shareholders' equity	259,969	256,077
Eliminations [Member]
Current Assets:
Cash
Intercompany receivable	(116,597)	(115,300)
Total current assets	(116,597)	(115,300)
Investment in subsidiaries	(102,124)	(96,706)
Total assets	(218,721)	(212,006)
Current Liabilities:
Intercompany payable	(116,597)	(115,300)
Total current liabilities	(116,597)	(115,300)
Shareholders' Equity:
Common stock and paid-in capital	(2,674)	(2,674)
Retained earnings	(99,450)	(94,032)
Total shareholders' equity	(102,124)	(96,706)
Total liabilities and shareholders' equity	$ (218,721)	$ (212,006)