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Retirement Benefit Plan (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended
--- | ---
Dec. 31, 2012 | Dec. 31, 2011 | Dec. 31, 2010
Retirement Benefit Plan |  |  |
Employer match of employee contributions of first 1% of eligible compensation (as a percent) | 100.00% |  |
Percentage of eligible compensation, matched 100% by employer | 1.00% |  |
Employer match of employee contributions of next 5% of eligible compensation (as a percent) | 50.00% |  |
Percentage of eligible compensation, matched 50% by employer | 5.00% |  |
Recognized matching contributions from retirement benefit plans (in dollars) | $ 7.6 | $ 8.7 | $ 3.9

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- Definition Represents the first level of employee contributions (percentage of compensation), which are matched by the employer. + References No definition available. + Details
Name: | exh_DefinedContributionPlanEmployerMatchEmployeeContributionLevelOne
Namespace Prefix: | exh_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Definition Represents the second level of employee contributions (percentage of compensation), which are matched by the employer. + References No definition available. + Details
Name: | exh_DefinedContributionPlanEmployerMatchEmployeeContributionLevelTwo
Namespace Prefix: | exh_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Represents the employer matching contribution of the first level of employee contributions. + References No definition available. + Details
Name: | exh_DefinedContributionPlanEmployerMatchLevelOne
Namespace Prefix: | exh_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Represents the employer matching contribution of the second level of employee contributions. + References No definition available. + Details
Name: | exh_DefinedContributionPlanEmployerMatchLevelTwo
Namespace Prefix: | exh_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | us-gaap_CompensationAndRetirementDisclosureAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition The amount of the cost recognized during the period for defined contribution plans. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 70 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6414718&loc=d3e28014-114942 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 132R -Paragraph 11 -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_DefinedContributionPlanCostRecognized
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration