Nature of Business (Details) $ in Billions	Dec. 31, 2017 USD ($)
Nature of Business [Abstract]
Direct domestic primary insurance in force	$ 194.9
Direct domestic primary risk in force	$ 50.3

The principal balance in a Company's records of all mortgage loans it insures.

No definition available.

Name:	mtg_DirectDomesticPrimaryInsuranceInForce
Namespace Prefix:	mtg_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant