Commitments (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Commitments and Contingencies
Contractual Obligation, Due in Next Twelve Months	$ 127,753,000
Contractual Obligation, Due in Second Year	111,674,000
Contractual Obligation, Due in Third Year	99,854,000
Contractual Obligation, Due in Fourth Year	102,540,000
Contractual Obligation, Due in Fifth Year	101,456,000
Contractual Obligation, Due after Fifth Year	469,348,000
Commitment amount	1,012,625,000
Operating Leases, Rent Expense	5,400,000	3,700,000	2,700,000
Drilling Rig Leasing Contracts
Commitments and Contingencies
Long-term Purchase Commitment, Amount	91,400,000
Gas gathering through-put commitments
Commitments and Contingencies
Long-term Purchase Commitment, Amount	858,700,000
Office Space Leases
Commitments and Contingencies
Long-term Purchase Commitment, Amount	55,300,000
Other miscellaneous contracts and leases
Commitments and Contingencies
Long-term Purchase Commitment, Amount	7,200,000
Asset under Construction
Commitments and Contingencies
Long-term Purchase Commitment, Amount	$ 67,000,000
Natural Gas [Member]
Commitments and Contingencies
Oil and Gas Delivery Commitments and Contracts, Remaining Contractual Volume	1,514,900,000,000
Crude Oil [Member]
Commitments and Contingencies
Oil and Gas Delivery Commitments and Contracts, Remaining Contractual Volume	36,000,000