Natural Gas Producing Activities (Unaudited) (Details) (USD $)	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Capitalized costs oil and gas producing activities net
Capitalized costs	$ 5,772,083,000	$ 4,655,217,000	$ 3,423,068,000
Accumulated depreciation and depletion	1,177,526,000	967,473,000	797,303,000
Net capitalized costs	4,594,557,000	3,687,744,000	2,625,765,000
Property acquisition:
Proved properties	108,717,000	15,359,000	6,035,000
Unproved properties	41,085,000	342,372,000	24,941,000
Exploration	2,344,000	5,105,000	14,909,000
Development	928,294,000	881,331,000	676,121,000
Liabilities assumed in exchange for proved developed properties	92,600,000
Revenues:
Affiliated	6,225,000	7,371,000	6,923,000
Nonaffiliated	785,060,000	530,286,000	414,067,000
Production costs	80,911,000	67,414,000	62,978,000
Exploration costs	4,932,000	5,368,000	17,905,000
Depreciation, depletion and accretion	257,144,000	183,699,000	117,424,000
Income tax expense	174,835,000	106,847,000	84,620,000
Results of operations from producing activities (excluding corporate overhead)	$ 273,463,000	$ 174,329,000	$ 138,063,000