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Other information about investments (Details 11) - EUR (€)
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Disclosure of subsidiaries [line items]    
Share capital € 4,005,000,000 € 4,005,000,000
Italy [Member] | Other Activities [Member] | Subsidiaries One [Member]    
Disclosure of subsidiaries [line items]    
Company name Filatura Tessile Nazionale Italiana - FILTENI SpA (in liquidation)  
Registered office Ferrandina (MT)  
Country of operation Italy  
Share capital € 4,644,000  
Consolidation or valutation method [1] Co.  
Italy [Member] | Other Activities [Member] | Subsidiaries One [Member] | Syndial SpA [Member]    
Disclosure of subsidiaries [line items]    
Shareholders Syndial SpA  
% Ownership [2] 59.56%  
Italy [Member] | Other Activities [Member] | Subsidiaries One [Member] | Third parties [Member]    
Disclosure of subsidiaries [line items]    
Shareholders Third parties  
% Ownership 40.44%  
Italy [Member] | Other Activities [Member] | Subsidiaries Two [Member]    
Disclosure of subsidiaries [line items]    
Company name Ottana Sviluppo ScpA (in liquidation)  
Registered office Nuoro  
Country of operation Italy  
Share capital € 516,000  
Consolidation or valutation method [1] Eq.  
Italy [Member] | Other Activities [Member] | Subsidiaries Two [Member] | Syndial SpA [Member]    
Disclosure of subsidiaries [line items]    
Shareholders Syndial SpA  
% Ownership 30.00%  
Italy [Member] | Other Activities [Member] | Subsidiaries Two [Member] | Third parties [Member]    
Disclosure of subsidiaries [line items]    
Shareholders Third parties  
% Ownership 70.00%  
Italy [Member] | Other Activities [Member] | Subsidiaries Three [Member]    
Disclosure of subsidiaries [line items]    
Company name [1],[3] Saipem SpA(#) (†)  
Registered office San Donato Milanese (MI)  
Country of operation Italy  
Share capital € 2,191,384,693  
Consolidation or valutation method [1] Eq.  
Italy [Member] | Other Activities [Member] | Subsidiaries Three [Member] | Third parties [Member]    
Disclosure of subsidiaries [line items]    
Shareholders Third parties  
% Ownership 67.98%  
Italy [Member] | Other Activities [Member] | Subsidiaries Three [Member] | Eni SpA [Member]    
Disclosure of subsidiaries [line items]    
Shareholders Eni SpA  
% Ownership [4] 30.54%  
Italy [Member] | Other Activities [Member] | Subsidiaries Three [Member] | Saipem SpA [Member]    
Disclosure of subsidiaries [line items]    
Shareholders Saipem SpA  
% Ownership 1.48%  
[1] Jointly controlled entity.
[2] Controlling interest: Syndial SpA 48.00 Third parties 52.00
[3] Company with shares quoted in the regulated market of Italy or of other EU countries
[4] Controlling interest: Eni SpA 31.00 Third parties 69.00