Oil And Natural Gas Properties (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013
Oil And Natural Gas Properties [Line Items]
Discount rate used to calculate ceiling, percentage	10.00%
Effects of cash flow hedges	$ 21.7
Impairment of Oil and Gas Properties without Impact of Hedges (Not taken)	17.7
Impairment of Oil and Gas Properties without Impact of Hedges, net of tax (Not taken)	$ 11.0
Apr'13 - Dec'13 [Member]
Oil And Natural Gas Properties [Line Items]
Oil and gas production volume covered by hedges that affect ceiling determination, MMBoe	6.9