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Cover Page - shares
9 Months Ended
Sep. 30, 2019
Oct. 31, 2019
Cover page.    
Document Type 10-Q  
Document Quarterly Report true  
Document Period End Date Sep. 30, 2019  
Document Transition Report false  
Entity File Number 001-32236  
Entity Registrant Name COHEN & STEERS INC  
Entity Incorporation, State or Country Code DE  
Entity Tax Identification Number 14-1904657  
Entity Address, Address Line One 280 Park Avenue  
Entity Address, State or Province NY  
Entity Address, City or Town NY  
Entity Address, Postal Zip Code 10017  
City Area Code 212  
Local Phone Number 832-3232  
Title of 12(b) Security Common Stock  
Trading Symbol CNS  
Security Exchange Name NYSE  
Entity Current Reporting Status Yes  
Entity Interactive Data Current Yes  
Entity Filer Category Large Accelerated Filer  
Entity Small Business false  
Entity Emerging Growth Company false  
Entity Shell Company false  
Entity Common Stock, Shares Outstanding   47,248,129
Entity Central Index Key 0001284812  
Current Fiscal Year End Date --12-31  
Document Fiscal Year Focus 2019  
Document Fiscal Period Focus Q3  
Amendment Flag false