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Income Taxes - Summary of Consolidated Federal Income Tax Return (Details) (USD $) | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Summary of consolidated Federal income tax return |  |  |
Current city income tax provision | $ 935,612 dhil_CurrentLocalTaxExpenseBenefit | $ 952,957 dhil_CurrentLocalTaxExpenseBenefit | $ 406,814 dhil_CurrentLocalTaxExpenseBenefit
Current state income tax provision | 429,147 dhil_CurrentStateTaxExpenseBenefit | 268,920 dhil_CurrentStateTaxExpenseBenefit | 122,704 dhil_CurrentStateTaxExpenseBenefit
Current federal income tax provision | 15,015,813 us-gaap_CurrentFederalTaxExpenseBenefit | 17,866,911 us-gaap_CurrentFederalTaxExpenseBenefit | 8,990,777 us-gaap_CurrentFederalTaxExpenseBenefit
Deferred federal income tax benefit | 2,404,433 us-gaap_DeferredFederalIncomeTaxExpenseBenefit | (5,611,451)us-gaap_DeferredFederalIncomeTaxExpenseBenefit | (368,572)us-gaap_DeferredFederalIncomeTaxExpenseBenefit
Provision for income taxes | $ 18,785,005 us-gaap_IncomeTaxExpenseBenefit | $ 13,477,337 us-gaap_IncomeTaxExpenseBenefit | $ 9,151,723 us-gaap_IncomeTaxExpenseBenefit

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