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Summary of Significant Accounting Policies - Risk of concentrations of credit, significant customers and significant suppliers (Details)

v3.21.2
Summary of Significant Accounting Policies - Risk of concentrations of credit, significant customers and significant suppliers (Details) - Evolv Technologies Holdings, Inc.
3 Months Ended 6 Months Ended 12 Months Ended
Jun. 30, 2021
Jun. 30, 2020
Jun. 30, 2021
Jun. 30, 2020
Dec. 31, 2020
Dec. 31, 2019
Total revenue | Customer concentration risk | Customer Representing More Than Ten Percentage            
Concentration Risk [Line Items]            
Concentration risk, percentage         10.70% 59.50%
Total revenue | Customer concentration risk | Customer A            
Concentration Risk [Line Items]            
Concentration risk, percentage         10.70%  
Total revenue | Customer concentration risk | Customer B            
Concentration Risk [Line Items]            
Concentration risk, percentage           26.60%
Total revenue | Customer concentration risk | Customer C            
Concentration Risk [Line Items]            
Concentration risk, percentage           21.70%
Total revenue | Customer concentration risk | Customer D            
Concentration Risk [Line Items]            
Concentration risk, percentage           11.20%
Total revenue | Credits Concentration Risk [Member] | Customer Representing More Than Ten Percentage            
Concentration Risk [Line Items]            
Concentration risk, percentage 17.00% 11.00% 14.00% 11.00%    
Total revenue | Credits Concentration Risk [Member] | Customer A            
Concentration Risk [Line Items]            
Concentration risk, percentage   11.00%   11.00%    
Total revenue | Credits Concentration Risk [Member] | Customer B            
Concentration Risk [Line Items]            
Concentration risk, percentage 17.00%          
Total revenue | Credits Concentration Risk [Member] | Customer H            
Concentration Risk [Line Items]            
Concentration risk, percentage     14.00%      
Accounts receivable | Credits Concentration Risk [Member] | Customer Representing More Than Ten Percentage            
Concentration Risk [Line Items]            
Concentration risk, percentage     30.10%   47.20%  
Accounts receivable | Credits Concentration Risk [Member] | Customer B            
Concentration Risk [Line Items]            
Concentration risk, percentage     11.80%      
Accounts receivable | Credits Concentration Risk [Member] | Customer C            
Concentration Risk [Line Items]            
Concentration risk, percentage     18.30%      
Accounts receivable | Credits Concentration Risk [Member] | Customer D            
Concentration Risk [Line Items]            
Concentration risk, percentage         23.80%  
Accounts receivable | Credits Concentration Risk [Member] | Customer E            
Concentration Risk [Line Items]            
Concentration risk, percentage         23.40%