Condensed Consolidating Financial Information (Condensed Consolidating Balance Sheets) (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2012	Dec. 31, 2011
Cash and cash equivalents	$ 17,508	$ 53,583	$ 206,167	$ 15,627
Restricted cash	7,108	8,542
Accounts receivable	387,324	377,638
Inventories	30,408	28,141
Other current assets	174,459	341,008
Total current assets	616,807	808,912
Property and equipment	13,561,738	13,028,439
Less: Accumulated depreciation, depletion and amortization	(7,380,230)	(7,191,463)
Total property and equipment, net	6,181,508	5,836,976
Other assets	118,476	91,639
TOTAL ASSETS	6,916,791	6,737,527	8,554,161
Accounts payable	616,135	556,980
Other current liabilities	115,339	210,791
Total current liabilities	731,474	767,771
Long-term debt	1,703,403	1,668,273
Deferred income taxes	1,131,312	1,049,138
Other liabilities	268,794	216,473
Total liabilities	3,834,983	3,701,655
Total Equity	3,081,808	3,035,872
TOTAL LIABILITIES AND EQUITY	6,916,791	6,737,527
Parent [Member]
Cash and cash equivalents	17,379	47,491	205,415	14,711
Restricted cash	7,108	8,542
Accounts receivable	2,720	2,677
Inventories		2
Other current assets	14,611	7,461
Total current assets	41,818	66,173
Intercompany receivables	2,277,188	2,259,713
Property and equipment	225,056	220,837
Less: Accumulated depreciation, depletion and amortization	(88,806)	(82,178)
Total property and equipment, net	136,250	138,659
Investments in subsidiaries (equity method)	2,344,562	2,309,947
Other assets	35,898	35,136
TOTAL ASSETS	4,835,716	4,809,628
Accounts payable	99,746	140,367
Other current liabilities	3,759	3,758
Total current liabilities	103,505	144,125
Long-term debt	1,703,403	1,668,273
Deferred income taxes	(116,029)	(116,207)
Other liabilities	63,029	77,565
Total liabilities	1,753,908	1,773,756
Total Equity	3,081,808	3,035,872
TOTAL LIABILITIES AND EQUITY	4,835,716	4,809,628
Guarantors [Member]
Cash and cash equivalents		5,988	489
Restricted cash
Accounts receivable	367,636	353,607
Inventories	29,117	26,975
Other current assets	153,233	321,396
Total current assets	549,986	707,966
Intercompany receivables	94	42
Property and equipment	11,979,897	11,491,222
Less: Accumulated depreciation, depletion and amortization	(7,090,914)	(6,923,106)
Total property and equipment, net	4,888,983	4,568,116
Other assets	68,058	42,247
TOTAL ASSETS	5,507,121	5,318,371
Accounts payable	434,903	375,604
Other current liabilities	109,036	205,623
Total current liabilities	543,939	581,227
Intercompany payables	2,254,521	2,108,360
Deferred income taxes	874,991	820,279
Other liabilities	191,521	124,505
Total liabilities	3,864,972	3,634,371
Total Equity	1,642,149	1,684,000
TOTAL LIABILITIES AND EQUITY	5,507,121	5,318,371
Non-Guarantors [Member]
Cash and cash equivalents	129	104	263	916
Restricted cash
Accounts receivable	16,968	21,354
Inventories	1,291	1,164
Other current assets	6,615	12,151
Total current assets	25,003	34,773
Intercompany receivables	26,325	27,077
Property and equipment	1,356,785	1,316,380
Less: Accumulated depreciation, depletion and amortization	(200,510)	(186,179)
Total property and equipment, net	1,156,275	1,130,201
Other assets	14,520	14,256
TOTAL ASSETS	1,222,123	1,206,307
Accounts payable	81,486	41,009
Other current liabilities	2,544	1,410
Total current liabilities	84,030	42,419
Intercompany payables	49,086	178,472
Deferred income taxes	372,350	345,066
Other liabilities	14,244	14,403
Total liabilities	519,710	580,360
Total Equity	702,413	625,947
TOTAL LIABILITIES AND EQUITY	1,222,123	1,206,307
Eliminations [Member]
Restricted cash
Intercompany receivables	(2,303,607)	(2,286,832)
Investments in subsidiaries (equity method)	(2,344,562)	(2,309,947)
TOTAL ASSETS	(4,648,169)	(4,596,779)
Intercompany payables	(2,303,607)	(2,286,832)
Total liabilities	(2,303,607)	(2,286,832)
Total Equity	(2,344,562)	(2,309,947)
TOTAL LIABILITIES AND EQUITY	$ (4,648,169)	$ (4,596,779)