XML 13 R1.htm IDEA: XBRL DOCUMENT v3.21.2
Document and Entity Information - shares
 9 Months Ended
Sep. 30, 2021
Nov. 02, 2021
Document and Entity Information    
Document Type 10-Q  
Document Quarterly Report true  
Document Transition Report false  
Document Period End Date Sep. 30, 2021  
Entity File Number 000-10537  
Entity Registrant Name OLD SECOND BANCORP INC  
Entity Incorporation, State or Country Code DE  
Entity Tax Identification Number 36-3143493  
Entity Address, Address Line One 37 South River Street  
Entity Address, City or Town Aurora  
Entity Address, State or Province IL  
Entity Address, Postal Zip Code 60507  
City Area Code (630)  
Local Phone Number 892-0202  
Title of 12(b) Security Common Stock  
Trading Symbol OSBC  
Security Exchange Name NASDAQ  
Entity Current Reporting Status Yes  
Entity Interactive Data Current Yes  
Entity Filer Category Accelerated Filer  
Entity Small Business false  
Entity Emerging Growth Company false  
Entity Shell Company false  
Entity Common Stock, Shares Outstanding   28,707,737
Current Fiscal Year End Date --12-31  
Document Fiscal Year Focus 2021  
Document Fiscal Period Focus Q3  
Entity Central Index Key 0000357173  
Amendment Flag false