Subsequent Events	9 Months Ended
Sep. 30, 2018
Subsequent Events [Abstract]
Subsequent Events	Subsequent Events On October 30, 2018, MGIC entered into a fully collateralized reinsurance agreement with Home Re 2018-1 Ltd. (“Home Re”), an unaffiliated special purpose insurer domiciled in Bermuda, that provides for up to $318.6 million of aggregate excess-of-loss reinsurance coverage as of August 1, 2018 on a portfolio of mortgage insurance policies having an insurance coverage in force date on or after July 1, 2016 and before January 1, 2018. For the reinsurance coverage period, MGIC will retain the first layer of $168.7 million of aggregate losses, and Home Re will then provide second layer coverage up to the outstanding reinsurance coverage amount. The reinsurance coverage premium due to Home Re is calculated by multiplying the outstanding reinsurance coverage amount at the beginning of a period by a coupon rate, which is the sum of one-month LIBOR plus a spread for each class of notes, and then subtracting actual investment income collected on the assets in the reinsurance trust during that period. The aggregate excess of loss reinsurance coverage decreases over a ten-year period, subject to certain conditions, as the underlying covered mortgages amortize, principal is prepaid, or mortgage insurance losses are paid. MGIC has rights to terminate the reinsurance agreement, which includes an option to terminate after seven years from the closing date. Home Re financed the coverage by issuing mortgage insurance-linked notes in an aggregate amount of $318.6 million to unaffiliated investors. The notes have ten-year legal maturities and are non-recourse to any assets of MGIC or its affiliates. The proceeds of the notes were deposited into a reinsurance trust for the benefit of MGIC that will be the source of reinsurance claim payments to MGIC and principal repayments on the mortgage insurance-linked notes. In connection with entering into the reinsurance agreement with Home Re, we concluded that the risk transfer requirements for reinsurance accounting were met as Home Re is assuming significant insurance risk and a reasonable possibility of significant loss. In addition, we assessed whether Home Re was a variable interest entity (“VIE”). A VIE is a legal entity that does not have sufficient equity at risk to finance its activities without additional subordinated financial support or is structured such that equity investors lack the ability to make sufficient decisions relating to the entity’s operations through voting rights or do not substantively participate in gains and losses of the entity. We concluded that Home Re is a VIE. However, given that MGIC (1) does not have the unilateral power to direct the activities that most significantly affect Home Re’s economic performance and (2) does not have the obligation to absorb losses or the right to receive benefits that could be potentially significant to Home Re, consolidation of Home Re is not required.

Subsequent Events

On October 30, 2018, MGIC entered into a fully collateralized reinsurance agreement with Home Re 2018-1 Ltd. (“Home Re”), an unaffiliated special purpose insurer domiciled in Bermuda, that provides for up to $318.6 million of aggregate excess-of-loss reinsurance coverage as of August 1, 2018 on a portfolio of mortgage insurance policies having an insurance coverage in force date on or after July 1, 2016 and before January 1, 2018. For the reinsurance coverage period, MGIC will retain the first layer of $168.7 million of aggregate losses, and Home Re will then provide second layer coverage up to the outstanding reinsurance coverage amount. The reinsurance coverage premium due to Home Re is calculated by multiplying the outstanding reinsurance coverage amount at the beginning of a period by a coupon rate, which is the sum of one-month LIBOR plus a spread for each class of notes, and then subtracting actual investment income collected on the assets in the reinsurance trust during that period. The aggregate excess of loss reinsurance coverage decreases over a ten-year period, subject to certain conditions, as the underlying covered mortgages amortize, principal is prepaid, or mortgage insurance losses are paid. MGIC has rights to terminate the reinsurance agreement, which includes an option to terminate after seven years from the closing date. Home Re financed the coverage by issuing mortgage insurance-linked notes in an aggregate amount of $318.6 million to unaffiliated investors. The notes have ten-year legal maturities and are non-recourse to any assets of MGIC or its affiliates. The proceeds of the notes were deposited into a reinsurance trust for the benefit of MGIC that will be the source of reinsurance claim payments to MGIC and principal repayments on the mortgage insurance-linked notes.

In connection with entering into the reinsurance agreement with Home Re, we concluded that the risk transfer requirements for reinsurance accounting were met as Home Re is assuming significant insurance risk and a reasonable possibility of significant loss. In addition, we assessed whether Home Re was a variable interest entity (“VIE”). A VIE is a legal entity that does not have sufficient equity at risk to finance its activities without additional subordinated financial support or is structured such that equity investors lack the ability to make sufficient decisions relating to the entity’s operations through voting rights or do not substantively participate in gains and losses of the entity. We concluded that Home Re is a VIE. However, given that MGIC (1) does not have the unilateral power to direct the activities that most significantly affect Home Re’s economic performance and (2) does not have the obligation to absorb losses or the right to receive benefits that could be potentially significant to Home Re, consolidation of Home Re is not required.