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Compensation Plans - Expense Recognition of Deferred Compensation (Detail) (Restricted Stock Units (RSUs), USD $)	Mar. 31, 2015
Restricted Stock Units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
2015 (Remaining Period)	$ 5,056,315dhil_EmployeeServiceShareBasedCompensationNonvestedAwardsCompensationCostNotYetRecognizedCurrent / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockUnitsRSUMember
2016	4,076,780dhil_EmployeeServiceShareBasedCompensationNonvestedAwardsCompensationCostNotYetRecognizedInYearOne / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockUnitsRSUMember
2017	3,548,508dhil_EmployeeServiceShareBasedCompensationNonvestedAwardsCompensationCostNotYetRecognizedInYearTwo / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockUnitsRSUMember
2018	2,845,415dhil_EmployeeServiceShareBasedCompensationNonvestedAwardsCompensationCostNotYetRecognizedInYearThree / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockUnitsRSUMember
2019	1,940,697dhil_EmployeeServiceShareBasedCompensationNonvestedAwardsCompensationCostNotYetRecognizedInYearFour / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockUnitsRSUMember
Thereafter	918,563dhil_EmployeeServiceShareBasedCompensationNonvestedAwardsCompensationCostNotYetRecognizedInYearFive / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockUnitsRSUMember
Total	$ 18,386,278us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockUnitsRSUMember

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- Definition

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- Definition

Unrecognized cost of unvested share-based compensation awards.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (i)

-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

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