Document and Entity Information - shares
6 Months Ended
Jun. 30, 2019
Jul. 31, 2019
Document And Entity Information [Abstract]    
Document Type 10-Q  
Amendment Flag false  
Document Period End Date Jun. 30, 2019  
Document Fiscal Year Focus 2019  
Document Fiscal Period Focus Q2  
Trading Symbol CNST  
Entity Central Index Key 0001434418  
Entity Current Reporting Status Yes  
Current Fiscal Year End Date --12-31  
Entity Filer Category Non-accelerated Filer  
Entity Shell Company false  
Entity Emerging Growth Company true  
Entity Ex Transition Period false  
Entity Small Business true  
Entity File Number 001-38584  
Entity Tax Identification Number 261741721  
Entity Address, Address Line One 215 First Street  
Entity Address, Address Line Two Suite 200  
Entity Address, City or Town Cambridge  
Entity Address, State or Province Massachusetts  
Entity Address, Postal Zip Code 02142  
City Area Code (617)  
Local Phone Number 714-0555  
Entity Common Stock, Shares Outstanding   25,822,696