Supplemental Information about Oil & Gas Producing Activities (Unaudited) (Costs Incurred in Oil and Gas Property Acquisition, Exploration, and Development Activities) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Costs Incurred, Acquisition of Oil and Natural Gas Properties
Proved properties	$ 70,700		$ 149,158		$ 334,409
Unproved properties	10,686		6,632		0
Interest Paid, Capitalized	17,915		29,117		28,321
Development	714,343		544,114		320,927
Exploration	16,405		627		2,310
Total	$ 812,134	[1]	$ 700,531	[1]	$ 657,646	[1]

[1] The total above does not reflect $17.9 million, $29.1 million and $28.3 million of capitalized interest incurred in 2012, 2011 and 2010, respectively.