Oil And Natural Gas Properties (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended	9 Months Ended
	Jun. 30, 2012	Sep. 30, 2013
Oil And Natural Gas Properties [Line Items]
Discount rate used to calculate ceiling, percentage		10.00%
Impairment of oil and gas properties	$ 115.9
Impairment of oil and gas properties, net of taxes	72.1
Effects of cash flow hedges	$ 32.5	$ 2.1
Jul'12 - Dec'12
Oil And Natural Gas Properties [Line Items]
Oil and gas production volume covered by hedges that affect ceiling determination, MMBoe	2.9
Jan'13 - Dec'13
Oil And Natural Gas Properties [Line Items]
Oil and gas production volume covered by hedges that affect ceiling determination, MMBoe	4.5
Oct’13 – Dec’13
Oil And Natural Gas Properties [Line Items]
Oil and gas production volume covered by hedges that affect ceiling determination, MMBoe		1.7