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Acquisitions of Ownership Interests in Partner Companies and Funds (Detail) (USD $)
3 Months Ended 16 Months Ended 1 Months Ended 37 Months Ended 15 Months Ended 99 Months Ended 25 Months Ended 1 Months Ended 23 Months Ended 1 Months Ended 6 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Dec. 31, 2014
Mar. 31, 2015
Feb. 28, 2015
Feb. 28, 2015
Feb. 28, 2015
Dec. 31, 2014
Feb. 28, 2015
Jan. 31, 2015
Jan. 31, 2015
Dec. 31, 2014
Schedule of Equity Method Investments [Line Items]                        
Payments to acquire equity method investments $ 14,129,000us-gaap_PaymentsToAcquireEquityMethodInvestments $ 5,361,000us-gaap_PaymentsToAcquireEquityMethodInvestments                    
Equity method investments 141,229,000us-gaap_EquityMethodInvestments   135,989,000us-gaap_EquityMethodInvestments 141,229,000us-gaap_EquityMethodInvestments       135,989,000us-gaap_EquityMethodInvestments       135,989,000us-gaap_EquityMethodInvestments
Quantia [Member]                        
Schedule of Equity Method Investments [Line Items]                        
Convertible bridge loan 1,700,000sfe_ConvertibleBridgeLoan
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= sfe_QuantiaMember
                     
Payments to acquire equity method investments     12,500,000us-gaap_PaymentsToAcquireEquityMethodInvestments
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= sfe_QuantiaMember
                 
Spongecell [Member]                        
Schedule of Equity Method Investments [Line Items]                        
Convertible bridge loan       4,000,000sfe_ConvertibleBridgeLoan
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= sfe_SpongecellMember
               
Payments to acquire equity method investments         10,000,000us-gaap_PaymentsToAcquireEquityMethodInvestments
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= sfe_SpongecellMember
             
Dabo [Member]                        
Schedule of Equity Method Investments [Line Items]                        
Convertible bridge loan       300,000sfe_ConvertibleBridgeLoan
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= sfe_DaboMember
               
Payments to acquire cost method investments           2,000,000us-gaap_PaymentsToAcquireOtherInvestments
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= sfe_DaboMember
           
Equity method investments 0us-gaap_EquityMethodInvestments
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= sfe_DaboMember
    0us-gaap_EquityMethodInvestments
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= sfe_DaboMember
               
Advantedge [Member]                        
Schedule of Equity Method Investments [Line Items]                        
Convertible bridge loan       800,000sfe_ConvertibleBridgeLoan
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= sfe_AdvantedgeMember
               
Payments to acquire equity method investments             15,300,000us-gaap_PaymentsToAcquireEquityMethodInvestments
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= sfe_AdvantedgeMember
         
AppFirst, Inc. [Member]                        
Schedule of Equity Method Investments [Line Items]                        
Convertible bridge loan 800,000sfe_ConvertibleBridgeLoan
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= sfe_AppfirstMember
                     
Payments to acquire equity method investments               8,600,000us-gaap_PaymentsToAcquireEquityMethodInvestments
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= sfe_AppfirstMember
       
Clutch [Member]                        
Schedule of Equity Method Investments [Line Items]                        
Convertible bridge loan                 800,000sfe_ConvertibleBridgeLoan
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= sfe_ClutchMember
     
Payments to acquire equity method investments                   7,500,000us-gaap_PaymentsToAcquireEquityMethodInvestments
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= sfe_ClutchMember
   
CloudMine [Member]                        
Schedule of Equity Method Investments [Line Items]                        
Payments to acquire equity method investments                 2,900,000us-gaap_PaymentsToAcquireEquityMethodInvestments
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= sfe_CloudMineMember
     
Ownership interest under equity method, percentage         30.10%us-gaap_EquityMethodInvestmentOwnershipPercentage
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= sfe_CloudMineMember
30.10%us-gaap_EquityMethodInvestmentOwnershipPercentage
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= sfe_CloudMineMember
30.10%us-gaap_EquityMethodInvestmentOwnershipPercentage
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= sfe_CloudMineMember
  30.10%us-gaap_EquityMethodInvestmentOwnershipPercentage
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= sfe_CloudMineMember
     
Aventura [Member]                        
Schedule of Equity Method Investments [Line Items]                        
Payments to acquire equity method investments                     6,000,000us-gaap_PaymentsToAcquireEquityMethodInvestments
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= sfe_AventuraMember
 
Ownership interest under equity method, percentage                   19.90%us-gaap_EquityMethodInvestmentOwnershipPercentage
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= sfe_AventuraMember
19.90%us-gaap_EquityMethodInvestmentOwnershipPercentage
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= sfe_AventuraMember
 
Full Measure [Member]                        
Schedule of Equity Method Investments [Line Items]                        
Payments to acquire equity method investments                     4,000,000us-gaap_PaymentsToAcquireEquityMethodInvestments
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= sfe_FullMeasureMember
 
Ownership interest under equity method, percentage                   25.40%us-gaap_EquityMethodInvestmentOwnershipPercentage
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= sfe_FullMeasureMember
25.40%us-gaap_EquityMethodInvestmentOwnershipPercentage
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= sfe_FullMeasureMember
 
Trice [Member]                        
Schedule of Equity Method Investments [Line Items]                        
Payments to acquire equity method investments                     $ 1,100,000us-gaap_PaymentsToAcquireEquityMethodInvestments
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= sfe_TriceMember
$ 5,000,000us-gaap_PaymentsToAcquireEquityMethodInvestments
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= sfe_TriceMember