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Stock-Based Compensation and Awards (Details 4) (USD $) In Millions, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2014
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)	1,672,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber / us-gaap_AwardTypeAxis = exh_RestrictedStockRestrictedStockUnitsPerformanceUnitsCashSettledRestrictedStockUnitsAndCashSettledPerformanceUnitsMember
Granted (in shares)	459,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod / us-gaap_AwardTypeAxis = exh_RestrictedStockRestrictedStockUnitsPerformanceUnitsCashSettledRestrictedStockUnitsAndCashSettledPerformanceUnitsMember
Vested (in shares)	(897,000)us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod / us-gaap_AwardTypeAxis = exh_RestrictedStockRestrictedStockUnitsPerformanceUnitsCashSettledRestrictedStockUnitsAndCashSettledPerformanceUnitsMember
Cancelled (in shares)	(64,000)us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod / us-gaap_AwardTypeAxis = exh_RestrictedStockRestrictedStockUnitsPerformanceUnitsCashSettledRestrictedStockUnitsAndCashSettledPerformanceUnitsMember
Non-vested awards at the end of the period (in shares)	1,170,000us-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)	$ 18.97us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue / us-gaap_AwardTypeAxis = exh_RestrictedStockRestrictedStockUnitsPerformanceUnitsCashSettledRestrictedStockUnitsAndCashSettledPerformanceUnitsMember
Granted (in dollars per share)	$ 41.35us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue / us-gaap_AwardTypeAxis = exh_RestrictedStockRestrictedStockUnitsPerformanceUnitsCashSettledRestrictedStockUnitsAndCashSettledPerformanceUnitsMember
Vested (in dollars per share)	$ 18.98us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue / us-gaap_AwardTypeAxis = exh_RestrictedStockRestrictedStockUnitsPerformanceUnitsCashSettledRestrictedStockUnitsAndCashSettledPerformanceUnitsMember
Cancelled (in dollars per share)	$ 25.86us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue / us-gaap_AwardTypeAxis = exh_RestrictedStockRestrictedStockUnitsPerformanceUnitsCashSettledRestrictedStockUnitsAndCashSettledPerformanceUnitsMember
Non-vested awards at the end of the period (in dollars per share)	$ 27.37us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue / us-gaap_AwardTypeAxis = exh_RestrictedStockRestrictedStockUnitsPerformanceUnitsCashSettledRestrictedStockUnitsAndCashSettledPerformanceUnitsMember
Unrecognized compensation
Expected unrecognized compensation cost related to unvested awards (in dollars)	$ 19.6us-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	1 year 8 months 12 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)	208,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber / us-gaap_AwardTypeAxis = exh_CashSettledRestrictedStockUnitsAndCashSettledPerformanceUnitsMember
Restricted shares, restricted stock units and performance units
Shares
Non-vested awards at the end of the period (in shares)	962,000us-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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Unrecognized cost of unvested share-based awards, other than options, awarded to employees as compensation.

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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 number of grants made 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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The weighted average fair value of nonvested awards on equity-based plans excluding option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, revenue or profit achievement stock award plan) for which the employer is contingently obligated to issue equity instruments or transfer assets to an employee who has not yet satisfied service or performance criteria necessary to gain title to proceeds from the sale of the award or underlying shares or units.

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