Acquisitions (Narrative) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		3 Months Ended
Mar. 31, 2013 Prize [Member]	Apr. 20, 2012 Prize [Member]	Mar. 31, 2013 East Texas Oil Field Properties [Member]	Dec. 04, 2012 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

Amount of cash paid to acquire the entity.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 141R

-Paragraph 68

-Subparagraph f(1)

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 141

-Paragraph 51

-Subparagraph d

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Name:	us-gaap_BusinessAcquisitionCostOfAcquiredEntityCashPaid
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant