SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Income Taxes (Detail) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES
Excess tax benefits recognized	$ 3.0	$ 0.4	$ 6.0	$ 3.9

Amount of realized tax benefit (shortfall) related to deductible compensation cost reported on the entity's tax return for equity instruments in excess of the compensation cost for those instruments recognized for financial reporting purposes.

No definition available.

Name:	wd_ExcessTaxBenefitShortfallRecognizedShareBasedCompensation
Namespace Prefix:	wd_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration