v3.19.3
Document and Entity Information - shares
9 Months Ended
Sep. 30, 2019
Nov. 01, 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 1-10816  
Entity Registrant Name MGIC Investment Corp  
Entity Incorporation, State or Country Code WI  
Entity Tax Identification Number 39-1486475  
Entity Address, Address Line One 250 E. Kilbourn Avenue  
Entity Address, City or Town Milwaukee,  
Entity Address, State or Province WI  
Entity Address, Postal Zip Code 53202  
City Area Code (414)  
Local Phone Number 347-6480  
Title of each class Common stock  
Trading Symbol MTG  
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   348,709,277
Entity Central Index Key 0000876437  
Current Fiscal Year End Date --12-31  
Document Fiscal Year Focus 2019  
Document Fiscal Period Focus Q3  
Amendment Flag false