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Income Taxes (Details 8) (USD $)
In Millions, unless otherwise specified
 12 Months Ended 1 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Sep. 30, 2014
Dec. 31, 2012
Oct. 31, 2012
Unrecognized tax benefits , which if recognized would affect the effective tax rate $ 14.6us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate 11.3us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate   $ 9.6us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate  
Potential interest expense and penalties related to unrecognized tax benefits 3.3us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestAccrued 3.4us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestAccrued   2.4us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestAccrued  
Aggregate over assessment on income tax examination         0.9us-gaap_IncomeTaxExaminationIncreaseDecreaseInLiabilityFromPriorYear
Amount of disallowance of telephone excise tax refund claims for which protests were filed with the Appeals Division of the IRS         0.5exh_IncomeTaxExaminationAmountForWhichProtestsWereFiledWithAppealsDivision
Refund of over-assessment received     0.9us-gaap_IncomeTaxExaminationLiabilityRefundAdjustmentFromSettlementWithTaxingAuthority    
State income tax returns, period subject to examination, low end of range 3 years        
State income tax returns, period subject to examination, high end of range 5 years        
State income tax examination period after formal notification, maximum 1 year        
Maximum reasonably possible decrease in unrecognized tax benefits $ 0.1us-gaap_SignificantChangeInUnrecognizedTaxBenefitsIsReasonablyPossibleEstimatedRangeOfChangeUpperBound        
IRS | Minimum          
Percentage of telephone excise tax refund claims settled   90.00%exh_PercentageOfTelephoneExciseTaxRefundClaimsSettled
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