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Supplemental Guarantor Financial Information (Details) (USD $)
In Millions, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Nov. 30, 2010
7.25% senior notes due December 2018      
Long-Term Debt      
Aggregate principal amount $ 350.0us-gaap_DebtInstrumentFaceAmount
/ us-gaap_LongtermDebtTypeAxis
= exh_SeniorNotesDueDecember2018Member
  $ 350.0us-gaap_DebtInstrumentFaceAmount
/ us-gaap_LongtermDebtTypeAxis
= exh_SeniorNotesDueDecember2018Member
Interest rate (as a percent) 7.25%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_LongtermDebtTypeAxis
= exh_SeniorNotesDueDecember2018Member
7.25%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_LongtermDebtTypeAxis
= exh_SeniorNotesDueDecember2018Member
7.25%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_LongtermDebtTypeAxis
= exh_SeniorNotesDueDecember2018Member
Guarantor Subsidiaries      
Long-Term Debt      
Ownership interest (as a percent) 100.00%us-gaap_MinorityInterestOwnershipPercentageByParent
/ dei_LegalEntityAxis
= us-gaap_GuarantorSubsidiariesMember