Segment Information - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
Mar. 31, 2015 Segment	Dec. 31, 2014
Segment Reporting [Abstract]
Number of reportable segments (segments)	4us-gaap_NumberOfReportableSegments
Investable assets	$ 2,506.9stfc_InvestableAssets	$ 2,444.2stfc_InvestableAssets

Number of segments reported by the entity. A reportable segment is a component of an entity for which there is an accounting requirement to report separate financial information on that component in the entity's financial statements.

No definition available.

Name:	us-gaap_NumberOfReportableSegments
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration