Oil and Gas Properties and Equipment (Details) (USD $)	3 Months Ended		12 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
Proven Properties [Abstract]
Acquisition, equipment, exploration, drilling and environmental costs	$ 7,320,488,000		$ 7,235,765,000
Capitalized Costs, Accumulated Depreciation, Depletion, Amortization and Valuation Allowance for Relating to Oil and Gas Producing Activities	5,636,792,000		5,578,265,000
Proved	1,683,696,000		1,657,500,000
Property, Plant and Equipment [Abstract]
Gathering Systems	283,885,000		282,879,000
Accumulated Depreciation Gathering Systems	(100,746,000)		(99,312,000)
Net Gathering Systems	183,139,000		183,567,000
Other Property And Equipment	14,779,000		14,772,000
Accumulated Depreciation Other Property And Equipment	(8,742,000)		(8,326,000)
Net Other Property And Equipment	6,037,000		6,446,000
Land	22,359,000		22,359,000
Net Capitalized Costs Property Plant And Equipment	211,535,000		212,372,000
Interest Expense, Borrowings	26,000,000	25,400,000
Initial Term Liquids Gathering System Lease	15
Interest Costs, Capitalized During Period	200,000	7,100,000
Ceiling Test Limitation Pretax Oil and Gas Properties			2,900,000,000
Other Asset Impairment Charges			92,500,000
Renewal Term Liquids Gathering Lease	5
Renewal Term Liquids Gathering System Lease Useful Life	75.00%
Annual Operating Lease Expense Liquids Gathering System	$ 20,000,000