CONDENSED CONSOLIDATING FINANCIAL INFORMATION (Details 2) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011	Mar. 31, 2011	Dec. 31, 2010
Current assets:
Cash and cash equivalents	$ 877	$ 3,012	$ 160,925	$ 217,569
Accounts receivable	72,734	79,089
Other current assets	147,741	128,571
Total current assets	221,352	210,672
Property and equipment, at cost	9,834,406	9,601,102
Less accumulated depreciation, depletion, and amortization	7,052,195	6,949,986
Net property and equipment	2,782,211	2,651,116
Investment in subsidiaries	0	0
Goodwill	239,420	239,420
Due from subsidiaries	0	0
Deferred income taxes	229,293	231,116
Other assets	58,998	48,827
TOTAL ASSETS	3,531,274	3,381,151
Current liabilities:
Accounts payable and accrued liabilities	224,502	247,880
Other current liabilities	139,808	92,957
Total current liabilities	364,310	340,837
Long-term debt	1,804,481	1,693,044
Due to parent and subsidiaries	0	0
Deferred income taxes	0	0
Other liabilities	160,079	154,157
Total liabilities	2,328,870	2,188,038
Shareholders' equity	1,202,404	1,193,113
TOTAL LIABILITIES AND SHAREHOLDERS' EQUITY	3,531,274	3,381,151
Parent Company
Current assets:
Cash and cash equivalents	677	1,734
Accounts receivable	35,769	43,999
Other current assets	146,754	127,667
Total current assets	183,200	173,400
Property and equipment, at cost	8,259,763	8,000,466
Less accumulated depreciation, depletion, and amortization	5,830,188	5,782,409
Net property and equipment	2,429,575	2,218,057
Investment in subsidiaries	139,518	160,591
Goodwill	216,460	216,460
Due from subsidiaries	149,564	214,394
Deferred income taxes	322,042	312,564
Other assets	58,998	48,827
TOTAL ASSETS	3,499,357	3,344,293
Current liabilities:
Accounts payable and accrued liabilities	216,006	235,788
Other current liabilities	133,409	86,618
Total current liabilities	349,415	322,406
Long-term debt	1,804,481	1,693,044
Due to parent and subsidiaries	0	0
Deferred income taxes	0	0
Other liabilities	143,057	135,730
Total liabilities	2,296,953	2,151,180
Shareholders' equity	1,202,404	1,193,113
TOTAL LIABILITIES AND SHAREHOLDERS' EQUITY	3,499,357	3,344,293
Guarantor Subsidiary
Current assets:
Cash and cash equivalents	4	1	0	3
Accounts receivable	36,164	34,142
Other current assets	314	313
Total current assets	36,482	34,456
Property and equipment, at cost	1,355,430	1,317,917
Less accumulated depreciation, depletion, and amortization	1,121,007	1,102,339
Net property and equipment	234,423	215,578
Investment in subsidiaries	0	0
Goodwill	22,960	22,960
Due from subsidiaries	47,875	46,944
Deferred income taxes	0	0
Other assets	0	0
TOTAL ASSETS	341,740	319,938
Current liabilities:
Accounts payable and accrued liabilities	4,039	8,846
Other current liabilities	88	63
Total current liabilities	4,127	8,909
Long-term debt	0	0
Due to parent and subsidiaries	0	0
Deferred income taxes	119,440	107,012
Other liabilities	2,745	2,614
Total liabilities	126,312	118,535
Shareholders' equity	215,428	201,403
TOTAL LIABILITIES AND SHAREHOLDERS' EQUITY	341,740	319,938
Combined Non-Guarantor Subsidiaries
Current assets:
Cash and cash equivalents	196	1,277	1,252	986
Accounts receivable	1,789	2,201
Other current assets	673	591
Total current assets	2,658	4,069
Property and equipment, at cost	219,213	282,719
Less accumulated depreciation, depletion, and amortization	101,000	65,238
Net property and equipment	118,213	217,481
Investment in subsidiaries	0	0
Goodwill	0	0
Due from subsidiaries	0	0
Deferred income taxes	26,691	25,564
Other assets	0	0
TOTAL ASSETS	147,562	247,114
Current liabilities:
Accounts payable and accrued liabilities	5,445	4,499
Other current liabilities	6,311	6,276
Total current liabilities	11,756	10,775
Long-term debt	0	0
Due to parent and subsidiaries	197,439	261,338
Deferred income taxes	0	0
Other liabilities	14,277	15,813
Total liabilities	223,472	287,926
Shareholders' equity	(75,910)	(40,812)
TOTAL LIABILITIES AND SHAREHOLDERS' EQUITY	147,562	247,114
Eliminations
Current assets:
Cash and cash equivalents	0	0
Accounts receivable	(988)	(1,253)
Other current assets	0	0
Total current assets	(988)	(1,253)
Property and equipment, at cost	0	0
Less accumulated depreciation, depletion, and amortization	0	0
Net property and equipment	0	0
Investment in subsidiaries	(139,518)	(160,591)
Goodwill	0	0
Due from subsidiaries	(197,439)	(261,338)
Deferred income taxes	(119,440)	(107,012)
Other assets	0	0
TOTAL ASSETS	(457,385)	(530,194)
Current liabilities:
Accounts payable and accrued liabilities	(988)	(1,253)
Other current liabilities	0	0
Total current liabilities	(988)	(1,253)
Long-term debt	0	0
Due to parent and subsidiaries	(197,439)	(261,338)
Deferred income taxes	(119,440)	(107,012)
Other liabilities	0	0
Total liabilities	(317,867)	(369,603)
Shareholders' equity	(139,518)	(160,591)
TOTAL LIABILITIES AND SHAREHOLDERS' EQUITY	$ (457,385)	$ (530,194)