Short-Duration Insurance Contracts Additional Information (Details)	12 Months Ended
Dec. 31, 2017
Short-Duration Insurance Contracts Additional Information [Abstract]
Unanticipated emergence from prior accident years	80.00%

Change in losses and loss expenses payable, unanticipated emergence from prior accident years

No definition available.

Name:	stfc_Unanticipatedemergencefromprioraccidentyears
Namespace Prefix:	stfc_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration