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Operating Leases - Additional Information (Detail)	12 Months Ended
	Mar. 31, 2013	Dec. 31, 2012	Jun. 30, 2013 sqft Location
Leases [Abstract]
Area of operating lease			25,500
Number of office space locations			2
Operating expenses	9.80	9.69

X

- Definition

Operating expenses per square foot.

+ References

No definition available.

+ Details

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Namespace Prefix:	dhil_
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Balance Type:	na
Period Type:	duration

X

- Definition

Area leased by company as of balance sheet date.

+ References

No definition available.

+ Details

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Namespace Prefix:	dhil_
Data Type:	num:areaItemType
Balance Type:	na
Period Type:	instant

X

- Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Definition

The number of units (items of property) under operating lease arrangements.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	instant