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Investments in Non-Consolidated Affiliates (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Investments in Non-Consolidated Affiliates
Late installment payment received from the sale of the joint venture	$ 5.0exh_EquityMethodInvestmentSalesProceedsInstallmentPaymentLatePayment
Installment payments, including annual charges, received from the sale of the joint venture		4.9exh_EquityMethodInvestmentSalesProceedsInstallmentPayment
Remaining principal amount due from sale of joint venture	$ 22exh_EquityMethodInvestmentExpectedNetSalesProceeds
PIGAP II
Ownership interest of each equity method investee
Ownership Interest (as a percent)	30.00%us-gaap_EquityMethodInvestmentOwnershipPercentage / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = exh_PIGPIIMember
El Furrial
Ownership interest of each equity method investee
Ownership Interest (as a percent)	33.30%us-gaap_EquityMethodInvestmentOwnershipPercentage / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = exh_ElFurrialMember

X

- Definition

The additional cash inflows expected to be received on disposal of an equity method investment.

+ References

No definition available.

+ Details

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X

- Definition

The cash inflows from installment payments received during the period on disposal of an equity method investment.

+ References

No definition available.

+ Details

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Namespace Prefix:	exh_
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Balance Type:	debit
Period Type:	duration

X

- Definition

The cash inflows from installment payments received as a late payment during the period on disposal of an equity method investment.

+ References

No definition available.

+ Details

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Namespace Prefix:	exh_
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X

- Definition

The percentage of ownership of common stock or equity participation in the investee accounted for under the equity method of accounting.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 323

-SubTopic 10

-Section 50

-Paragraph 3

-Subparagraph (a)(1)

-URI http://asc.fasb.org/extlink&oid=6382943&loc=d3e33918-111571

+ Details

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- Details

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- Details

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- Details

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- Details

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