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Retirement Benefit Plan (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Retirement Benefit Plan |  |  |
Employer match of employee contributions of first 1% of eligible compensation (as a percent) | 100.00%exh_DefinedContributionPlanEmployerMatchLevelOne |  |
Percentage of eligible compensation, matched 100% by employer | 1.00%exh_DefinedContributionPlanEmployerMatchEmployeeContributionLevelOne |  |
Employer match of employee contributions of next 5% of eligible compensation (as a percent) | 50.00%exh_DefinedContributionPlanEmployerMatchLevelTwo |  |
Percentage of eligible compensation, matched 50% by employer | 5.00%exh_DefinedContributionPlanEmployerMatchEmployeeContributionLevelTwo |  |
Recognized matching contributions from retirement benefit plans (in dollars) | $ 9.7 us-gaap_DefinedContributionPlanCostRecognized | $ 8.2 us-gaap_DefinedContributionPlanCostRecognized | $ 7.5 us-gaap_DefinedContributionPlanCostRecognized

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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 + Details
Name: | us-gaap_DefinedContributionPlanCostRecognized
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration