Acquisitions (Narrative) (Details) (USD $) In Millions, unless otherwise specified	1 Months Ended	6 Months Ended	1 Months Ended	3 Months Ended	6 Months Ended
	Apr. 30, 2012 Prize [Member]	Jun. 30, 2013 Prize [Member]	Dec. 31, 2012 East Texas Oil Field Properties [Member]	Mar. 31, 2013 East Texas Oil Field Properties [Member]	Jun. 30, 2013 East Texas Oil Field Properties [Member]
Business Acquisition [Line Items]
Date of acquisition		Apr. 20, 2012			Dec. 04, 2012
Cash paid for acquisition	$ 225		$ 214
Effective date of acquisition		Jan. 01, 2012			Nov. 01, 2012
Receivable received for purchase price adjustment				$ 2.3