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Retirement Benefit Plan (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	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)	$ 9.7us-gaap_DefinedContributionPlanCostRecognized	$ 8.2us-gaap_DefinedContributionPlanCostRecognized	$ 7.5us-gaap_DefinedContributionPlanCostRecognized

X

- 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

X

- 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

X

- Details

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

X

- 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