Oil and Gas Properties and Equipment (Details) (USD $)	6 Months Ended		12 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012
Proven Properties [Abstract]
Acquisition, equipment, exploration, drilling and environmental costs	$ 7,413,154,000		$ 7,235,765,000
Capitalized Costs, Accumulated Depreciation, Depletion, Amortization and Valuation Allowance for Relating to Oil and Gas Producing Activities	5,693,732,000		5,578,265,000
Proved	1,719,422,000		1,657,500,000
Property, Plant and Equipment [Abstract]
Gathering Systems	286,153,000		282,879,000
Accumulated Depreciation Gathering Systems	(102,223,000)		(99,312,000)
Net Gathering Systems	183,930,000		183,567,000
Other Property And Equipment	14,855,000		14,772,000
Accumulated Depreciation Other Property And Equipment	(9,056,000)		(8,326,000)
Net Other Property And Equipment	5,799,000		6,446,000
Land	22,360,000		22,359,000
Net Capitalized Costs Property Plant And Equipment	212,089,000		212,372,000
Interest Expense, Borrowings	51,400,000	51,100,000
Interest Costs, Capitalized During Period	400,000	14,000,000
Ceiling Test Limitation Pretax Oil and Gas Properties			2,900,000,000
Other Asset Impairment Charges			$ 92,500,000