Condensed Consolidating Financial Statements (Condensed Consolidating Balance Sheet) (Details) (USD $) In Thousands	Sep. 30, 2011	Dec. 31, 2010	Sep. 30, 2010	Dec. 31, 2009
Cash and cash equivalents	$ 56,418	$ 44,229	$ 51,124	$ 68,407
Restricted cash	117,339	161,717
Other current assets	436,408	314,514
Total current assets	610,165	520,460
Equity investments	287,979	379,001
Unproved oil and natural gas properties and development costs not being amortized	747,131	599,409
Proved developed and undeveloped oil and natural gas properties	3,342,961	2,370,962
Accumulated depletion	(1,552,174)	(1,312,216)
Oil and natural gas properties, net	2,537,918	1,658,155
Gas gathering, office and field equipment, net	152,465	176,306
Investments in and advances to affiliates	0	0
Deferred financing costs, net	31,518	30,704
Derivative financial instruments	23,356	23,722
Goodwill	218,256	218,256
Other assets	7,848	470,816
Total assets	3,869,505	3,477,420
Current liabilities	365,500	285,698
Long-term debt	1,712,555	1,588,269
Deferred income taxes	0	0
Other liabilities	63,651	62,901
Payable to parent	0	0
Total shareholders' equity	1,727,799	1,540,552	1,599,871	859,588
Total liabilities and shareholders' equity	3,869,505	3,477,420
Resources [Member]
Cash and cash equivalents	79,299	76,763	80,568	47,412
Restricted cash	0	0
Other current assets	133,990	83,913
Total current assets	213,289	160,676
Equity investments	0	0
Unproved oil and natural gas properties and development costs not being amortized	25,311	37,818
Proved developed and undeveloped oil and natural gas properties	444,887	385,357
Accumulated depletion	(314,659)	(295,453)
Oil and natural gas properties, net	155,539	127,722
Gas gathering, office and field equipment, net	28,808	28,837
Investments in and advances to affiliates	1,092,098	964,806
Deferred financing costs, net	31,518	30,704
Derivative financial instruments	17,229	13,665
Goodwill	38,100	38,100
Other assets	3	3
Total assets	1,576,584	1,364,513
Current liabilities	67,651	50,654
Long-term debt	1,712,555	1,588,269
Deferred income taxes	0	0
Other liabilities	7,721	10,234
Payable to parent	(1,939,142)	(1,825,196)
Total shareholders' equity	1,727,799	1,540,552
Total liabilities and shareholders' equity	1,576,584	1,364,513
Guarantor Subsidiaries [Member]
Cash and cash equivalents	(22,881)	(32,534)	(29,444)	20,995
Restricted cash	117,339	161,717
Other current assets	302,418	230,590
Total current assets	396,876	359,773
Equity investments	0	0
Unproved oil and natural gas properties and development costs not being amortized	721,820	561,591
Proved developed and undeveloped oil and natural gas properties	2,898,074	1,985,605
Accumulated depletion	(1,237,515)	(1,016,763)
Oil and natural gas properties, net	2,382,379	1,530,433
Gas gathering, office and field equipment, net	123,657	131,276
Investments in and advances to affiliates	0	92,973
Deferred financing costs, net	0	0
Derivative financial instruments	6,127	10,057
Goodwill	180,156	180,156
Other assets	7,845	470,813
Total assets	3,097,040	2,775,481
Current liabilities	297,849	228,332
Long-term debt	0	0
Deferred income taxes	0	0
Other liabilities	55,930	52,667
Payable to parent	1,945,298	1,821,530
Total shareholders' equity	797,963	672,952
Total liabilities and shareholders' equity	3,097,040	2,775,481
Non-Guarantor Subsidiaries [Member]
Cash and cash equivalents	0	0	0	0
Restricted cash	0	0
Other current assets	0	11
Total current assets	0	11
Equity investments	287,979	379,001
Unproved oil and natural gas properties and development costs not being amortized	0	0
Proved developed and undeveloped oil and natural gas properties	0	0
Accumulated depletion	0	0
Oil and natural gas properties, net	0	0
Gas gathering, office and field equipment, net	0	16,193
Investments in and advances to affiliates	0	0
Deferred financing costs, net	0	0
Derivative financial instruments	0	0
Goodwill	0	0
Other assets	0	0
Total assets	287,979	395,205
Current liabilities	0	6,712
Long-term debt	0	0
Deferred income taxes	0	0
Other liabilities	0	0
Payable to parent	(6,156)	3,666
Total shareholders' equity	294,135	384,827
Total liabilities and shareholders' equity	287,979	395,205
Eliminations [Member]
Cash and cash equivalents	0	0	0	0
Restricted cash	0	0
Other current assets	0	0
Total current assets	0	0
Equity investments	0	0
Unproved oil and natural gas properties and development costs not being amortized	0	0
Proved developed and undeveloped oil and natural gas properties	0	0
Accumulated depletion	0	0
Oil and natural gas properties, net	0	0
Gas gathering, office and field equipment, net	0	0
Investments in and advances to affiliates	(1,092,098)	(1,057,779)
Deferred financing costs, net	0	0
Derivative financial instruments	0	0
Goodwill	0	0
Other assets	0	0
Total assets	(1,092,098)	(1,057,779)
Current liabilities	0	0
Long-term debt	0	0
Deferred income taxes	0	0
Other liabilities	0	0
Payable to parent	0	0
Total shareholders' equity	(1,092,098)	(1,057,779)
Total liabilities and shareholders' equity	$ (1,092,098)	$ (1,057,779)