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Ownership Interests in and Advances to Partner Companies and Funds - Narrative (Detail) (USD $)
3 Months Ended 1 Months Ended 12 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Feb. 28, 2014
Jan. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2014
Investment [Line Items]              
Equity method investments $ 141,229,000us-gaap_EquityMethodInvestments           $ 135,989,000us-gaap_EquityMethodInvestments
Proceeds from sale of business 2,192,000us-gaap_ProceedsFromDivestitureOfBusinessesAndInterestsInAffiliates 77,187,000us-gaap_ProceedsFromDivestitureOfBusinessesAndInterestsInAffiliates          
Dabo [Member]              
Investment [Line Items]              
Asset impairment charges 2,300,000us-gaap_AssetImpairmentCharges
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= sfe_DaboMember
           
Equity method investments 0us-gaap_EquityMethodInvestments
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= sfe_DaboMember
           
Crescendo Bioscience, Inc. [Member]              
Investment [Line Items]              
Realized gains on cost method investment   27,400,000us-gaap_CostMethodInvestmentsRealizedGains
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= sfe_CrescendoMember
         
Gain on Escrow Proceed Receipts 2,000,000sfe_GainonEscrowProceedReceipts
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= sfe_CrescendoMember
           
Amount held in escrow 900,000sfe_AmountHeldInEscrow
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= sfe_CrescendoMember
           
Proceeds from sale of business     38,400,000us-gaap_ProceedsFromDivestitureOfBusinessesAndInterestsInAffiliates
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= sfe_CrescendoMember
       
Nupathe [Member]              
Investment [Line Items]              
Proceeds sale of business (in dollars per share)     $ 3.65sfe_ProceedsFromDivestitureofBusinessesandInterestsinAffiliatesperShare
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= sfe_NupatheMember
       
Proceeds from sale of business     23,100,000us-gaap_ProceedsFromDivestitureOfBusinessesAndInterestsInAffiliates
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= sfe_NupatheMember
       
Proceeds from milestone payments (in dollars per share)     $ 3.15sfe_ProceedsfromMilestonePaymentsperShare
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= sfe_NupatheMember
       
Proceeds from milestone payments     24,200,000sfe_ProceedsfromMilestonePayments
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= sfe_NupatheMember
       
Gain on sale of business   3,000,000us-gaap_GainLossOnSaleOfBusiness
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= sfe_NupatheMember
         
Alverix, Inc. [Member]              
Investment [Line Items]              
Amount held in escrow       1,700,000sfe_AmountHeldInEscrow
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= sfe_AlverixIncMember
     
Proceeds from sale of business       15,700,000us-gaap_ProceedsFromDivestitureOfBusinessesAndInterestsInAffiliates
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= sfe_AlverixIncMember
     
Gain on sale of business   15,700,000us-gaap_GainLossOnSaleOfBusiness
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= sfe_AlverixIncMember
         
Equity Income (Loss) [Member]              
Investment [Line Items]              
Immaterial prior period adjustment made in current period   $ 1,700,000sfe_Immaterialpriorperiodadjustmentmadeincurrentperiod
/ us-gaap_IncomeStatementLocationAxis
= sfe_EquityIncomeLossMember
    $ 1,400,000sfe_Immaterialpriorperiodadjustmentmadeincurrentperiod
/ us-gaap_IncomeStatementLocationAxis
= sfe_EquityIncomeLossMember
$ 300,000sfe_Immaterialpriorperiodadjustmentmadeincurrentperiod
/ us-gaap_IncomeStatementLocationAxis
= sfe_EquityIncomeLossMember