Premium Deficiency Reserve (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Premium Deficiency Reserve [Abstract]
Change in premium deficiency reserve	$ 6	$ 24	$ 19

Increase (decrease) in the present value of expected future paid losses and expenses that exceeded the present value of expected future premium to be collected and already established loss and loss adjustment expense reserves.

No definition available.

Name:	mtg_IncreaseDecreaseInPremiumDeficiencyReserve
Namespace Prefix:	mtg_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration