Nature of Business	12 Months Ended
Dec. 31, 2017
Nature of Business [Abstract]
Nature of Business	Nature of Business MGIC Investment Corporation is a holding company which, through Mortgage Guaranty Insurance Corporation ("MGIC"), is principally engaged in the mortgage insurance business. We provide mortgage insurance to lenders throughout the United States and to government sponsored entities to protect against loss from defaults on low down payment residential mortgage loans. Our principal product is primary mortgage insurance. Primary mortgage insurance provides mortgage default protection on individual loans and covers unpaid loan principal, delinquent interest and certain expenses associated with the default and subsequent foreclosure or sale approved by us. Through certain non-insurance subsidiaries, we also provide various services for the mortgage finance industry, such as contract underwriting, analysis of loan originations and portfolios, and mortgage lead generation. An insurance subsidiary of MGIC provides credit insurance for certain mortgages under Fannie Mae and Freddie Mac (the "GSEs") credit risk transfer programs in transactions entered into in 2016. At December 31, 2017, our direct domestic primary insurance in force ("IIF") was $194.9 billion, which represents the principal balance in our records of all mortgage loans that we insure, and our direct domestic primary risk in force ("RIF") was $50.3 billion, which represents the IIF multiplied by the insurance coverage percentage. Substantially all of our insurance written since 2008 has been for loans purchased by the GSEs. We operate under the Private Mortgage Insurer Eligibility Requirements ("PMIERs") of the GSEs that became effective December 31, 2015 and which have been amended from time to time. The financial requirements of the PMIERs require a mortgage insurer’s "Available Assets" (generally only the most liquid assets of an insurer) to equal or exceed its "Minimum Required Assets" (which are based on an insurer's book, calculated from tables of factors with several risk dimensions and subject to a floor amount). Based on our interpretation of the PMIERs, as of December 31, 2017, MGIC’s Available Assets are in excess of its Minimum Required Assets; and MGIC is in compliance with the financial requirements of the PMIERs and eligible to insure loans purchased by the GSEs.

Nature of Business

MGIC Investment Corporation is a holding company which, through Mortgage Guaranty Insurance Corporation ("MGIC"), is principally engaged in the mortgage insurance business. We provide mortgage insurance to lenders throughout the United States and to government sponsored entities to protect against loss from defaults on low down payment residential mortgage loans. Our principal product is primary mortgage insurance. Primary mortgage insurance provides mortgage default protection on individual loans and covers unpaid loan principal, delinquent interest and certain expenses associated with the default and subsequent foreclosure or sale approved by us. Through certain non-insurance subsidiaries, we also provide various services for the mortgage finance industry, such as contract underwriting, analysis of loan originations and portfolios, and mortgage lead generation. An insurance subsidiary of MGIC provides credit insurance for certain mortgages under Fannie Mae and Freddie Mac (the "GSEs") credit risk transfer programs in transactions entered into in 2016.

At December 31, 2017, our direct domestic primary insurance in force ("IIF") was $194.9 billion, which represents the principal balance in our records of all mortgage loans that we insure, and our direct domestic primary risk in force ("RIF") was $50.3 billion, which represents the IIF multiplied by the insurance coverage percentage.

Substantially all of our insurance written since 2008 has been for loans purchased by the GSEs. We operate under the Private Mortgage Insurer Eligibility Requirements ("PMIERs") of the GSEs that became effective December 31, 2015 and which have been amended from time to time. The financial requirements of the PMIERs require a mortgage insurer’s "Available Assets" (generally only the most liquid assets of an insurer) to equal or exceed its "Minimum Required Assets" (which are based on an insurer's book, calculated from tables of factors with several risk dimensions and subject to a floor amount). Based on our interpretation of the PMIERs, as of December 31, 2017, MGIC’s Available Assets are in excess of its Minimum Required Assets; and MGIC is in compliance with the financial requirements of the PMIERs and eligible to insure loans purchased by the GSEs.