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Commitments and Contingencies (Details 2) (USD $) In Millions, unless otherwise specified	3 Months Ended	15 Months Ended
	Mar. 31, 2013	Mar. 31, 2013
Ad valorem tax
Ad valorem tax benefit	$ 2.0	$ 8.8
Ad valorem tax benefit agreed to by a number of Appraisal Review Boards	1.5	1.5
Maximum
Business acquisition, contingent consideration
Contingent payments	$ 46	$ 46

X

- Definition

The amount of ad valorem tax benefit included as a component of cost of goods sold (excluding depreciation and amortization).

+ References

No definition available.

+ Details

Name:	exh_AdValoremTaxExpenseBenefit
Namespace Prefix:	exh_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- Definition

The amount of ad valorem tax benefit agreed to by one or more appraisal review boards.

+ References

No definition available.

+ Details

Name:	exh_AdValoremTaxExpenseBenefitAgreedToByAppraisalReviewBoards
Namespace Prefix:	exh_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

X

- Details

Name:	exh_OtherTaxAbstract
Namespace Prefix:	exh_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Details

Name:	us-gaap_BusinessAcquisitionContingentConsiderationLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Definition

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

+ References

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-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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.

+ Details

Name:	us-gaap_BusinessAcquisitionContingentConsiderationPotentialCashPayment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant