Operating Segments - Narrative (Detail)	Dec. 31, 2017 partner_company
Segment Reporting [Abstract]
Non-consolidated partner companies	25

Partner Companies Not Consolidated In Financial Statements.

No definition available.

Name:	sfe_PartnerCompaniesNotConsolidatedInFinancialStatements
Namespace Prefix:	sfe_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant