Commitments and Contingencies (Details 2) (USD $) In Millions, unless otherwise specified	6 Months Ended	18 Months Ended
Jun. 30, 2013	Jun. 30, 2013
Ad valorem tax
Ad valorem tax benefit	$ 4.0	$ 10.8
Ad valorem tax benefit agreed to by a number of appraisal review boards and county appraisal districts		2.3
Maximum
Business acquisition, contingent consideration
Contingent payments	$ 46	$ 46

Amount of potential cash payments that could result from the contingent consideration arrangement.

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 141

-Paragraph 51

-Subparagraph f

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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_BusinessAcquisitionContingentConsiderationPotentialCashPayment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant