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Stock-Based Compensation (Details 3) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2015
Restricted stock, restricted stock units, performance units, cash settled restricted stock units and cash settled performance units
Shares
Non-vested awards at the beginning of the period (in shares)	1,170,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber / us-gaap_AwardTypeAxis = exh_RestrictedStockRestrictedStockUnitsPerformanceUnitsCashSettledRestrictedStockUnitsAndCashSettledPerformanceUnitsMember
Granted (in shares)	676,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod / us-gaap_AwardTypeAxis = exh_RestrictedStockRestrictedStockUnitsPerformanceUnitsCashSettledRestrictedStockUnitsAndCashSettledPerformanceUnitsMember
Vested (in shares)	(695,000)us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod / us-gaap_AwardTypeAxis = exh_RestrictedStockRestrictedStockUnitsPerformanceUnitsCashSettledRestrictedStockUnitsAndCashSettledPerformanceUnitsMember
Cancelled (in shares)	(9,000)us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod / us-gaap_AwardTypeAxis = exh_RestrictedStockRestrictedStockUnitsPerformanceUnitsCashSettledRestrictedStockUnitsAndCashSettledPerformanceUnitsMember
Non-vested awards at the end of the period (in shares)	1,142,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber / us-gaap_AwardTypeAxis = exh_RestrictedStockRestrictedStockUnitsPerformanceUnitsCashSettledRestrictedStockUnitsAndCashSettledPerformanceUnitsMember
Weighted Average Grant-Date Fair Value Per Share
Non-vested awards at the beginning of the period (in dollars per share)	$ 27.37us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue / us-gaap_AwardTypeAxis = exh_RestrictedStockRestrictedStockUnitsPerformanceUnitsCashSettledRestrictedStockUnitsAndCashSettledPerformanceUnitsMember
Granted (in dollars per share)	$ 31.99us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue / us-gaap_AwardTypeAxis = exh_RestrictedStockRestrictedStockUnitsPerformanceUnitsCashSettledRestrictedStockUnitsAndCashSettledPerformanceUnitsMember
Vested (in dollars per share)	$ 23.23us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue / us-gaap_AwardTypeAxis = exh_RestrictedStockRestrictedStockUnitsPerformanceUnitsCashSettledRestrictedStockUnitsAndCashSettledPerformanceUnitsMember
Cancelled (in dollars per share)	$ 32.68us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue / us-gaap_AwardTypeAxis = exh_RestrictedStockRestrictedStockUnitsPerformanceUnitsCashSettledRestrictedStockUnitsAndCashSettledPerformanceUnitsMember
Non-vested awards at the end of the period (in dollars per share)	$ 32.58us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue / us-gaap_AwardTypeAxis = exh_RestrictedStockRestrictedStockUnitsPerformanceUnitsCashSettledRestrictedStockUnitsAndCashSettledPerformanceUnitsMember
Unrecognized compensation
Expected unrecognized compensation cost related to unvested awards (in dollars)	$ 34.7us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedShareBasedAwardsOtherThanOptions / us-gaap_AwardTypeAxis = exh_RestrictedStockRestrictedStockUnitsPerformanceUnitsCashSettledRestrictedStockUnitsAndCashSettledPerformanceUnitsMember
Weighted-average period over which the expected unrecognized compensation cost related to unvested stock options will be recognized	2 years 3 months 18 days
Restricted stock, restricted stock units, performance units, cash settled restricted stock units and cash settled performance units | First anniversary vesting
Stock-based compensation
Vesting percentage	33.33%us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage / us-gaap_AwardTypeAxis = exh_RestrictedStockRestrictedStockUnitsPerformanceUnitsCashSettledRestrictedStockUnitsAndCashSettledPerformanceUnitsMember / us-gaap_VestingAxis = us-gaap_ShareBasedCompensationAwardTrancheOneMember
Vesting period	1 year
Restricted stock, restricted stock units, performance units, cash settled restricted stock units and cash settled performance units | Second anniversary vesting
Stock-based compensation
Vesting percentage	33.33%us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage / us-gaap_AwardTypeAxis = exh_RestrictedStockRestrictedStockUnitsPerformanceUnitsCashSettledRestrictedStockUnitsAndCashSettledPerformanceUnitsMember / us-gaap_VestingAxis = us-gaap_ShareBasedCompensationAwardTrancheTwoMember
Vesting period	2 years
Restricted stock, restricted stock units, performance units, cash settled restricted stock units and cash settled performance units | Third anniversary vesting
Stock-based compensation
Vesting percentage	33.33%us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage / us-gaap_AwardTypeAxis = exh_RestrictedStockRestrictedStockUnitsPerformanceUnitsCashSettledRestrictedStockUnitsAndCashSettledPerformanceUnitsMember / us-gaap_VestingAxis = us-gaap_ShareBasedCompensationAwardTrancheThreeMember
Vesting period	3 years
Cash settled restricted stock units and cash settled performance units
Shares
Non-vested awards at the end of the period (in shares)	56,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber / us-gaap_AwardTypeAxis = exh_CashSettledRestrictedStockUnitsAndCashSettledPerformanceUnitsMember
Restricted stock shares, restricted stock units and performance units
Shares
Non-vested awards at the end of the period (in shares)	1,086,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber / us-gaap_AwardTypeAxis = exh_RestrictedStockRestrictedStockUnitsAndPerformanceUnitsMember

X

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Weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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The number of equity-based payment instruments, excluding stock (or unit) options, that were forfeited during the reporting period.

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Weighted average fair value as of the grant date of equity-based award plans other than stock (unit) option plans that were not exercised or put into effect as a result of the occurrence of a terminating event.

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The weighted average fair value at grant date for nonvested equity-based awards issued during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date.

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