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Long-Term Debt (Details 2) (Revolving credit facility due July 2016, USD $)
In Millions, unless otherwise specified
Mar. 31, 2015
Revolving credit facility due July 2016
 
Long-Term Debt  
Outstanding borrowings $ 354.0us-gaap_LineOfCredit
/ us-gaap_CreditFacilityAxis
= exh_RevolvingCreditFacilityDueJuly2016Member
Outstanding letters of credit 91.3us-gaap_LettersOfCreditOutstandingAmount
/ us-gaap_CreditFacilityAxis
= exh_RevolvingCreditFacilityDueJuly2016Member
Undrawn capacity under revolving credit facility 454.7us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity
/ us-gaap_CreditFacilityAxis
= exh_RevolvingCreditFacilityDueJuly2016Member
Available borrowings under revolving credit facility $ 454.7exh_LineOfCreditFacilityCurrentAvailableBorrowingCapacity
/ us-gaap_CreditFacilityAxis
= exh_RevolvingCreditFacilityDueJuly2016Member