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Shareholders' equity (Details Textual) - EUR (€) € / shares in Units, € in Millions |  | 12 Months Ended
--- | --- | ---
May 12, 2016 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Statement Line Items [Line Items] |  |  |  |
Share capital |  | € 4,005 | € 4,005 |
Dividends proposed or declared before financial statements authorised for issue but not recognised as distribution to owners per share | € 0.40 |  |  |
Dividends paid, ordinary shares per share |  | € 0.4 | € 0.4 | € 0.4
Capital redemption reserve |  | € 581 | € 581 |
Other reserves |  | € 42,490 | € 52,329 |
Number of shares outstanding |  | 33,045,197 | 33,045,197 |
Treasury shares |  | € 581 | € 581 |
Distributable Reserves |  | 43,200 |  |
parent company [Member] |  |  |  |
Statement Line Items [Line Items] |  |  |  |
Share capital |  | € 4,005,358,876 | € 4,005,358,876 |
Number of shares issued |  | 3,634,185,330 | 3,634,185,330 |
Tigaz Zrt [Member] |  |  |  |
Statement Line Items [Line Items] |  |  |  |
Other reserves |  | € 4 | € 4 |
Proportion of ownership interest in subsidiary |  | 48.55% | 48.55% |
Eni Gas Power France SA [Member] |  |  |  |
Statement Line Items [Line Items] |  |  |  |
Other reserves |  | € 124 | € 124 |
Proportion of ownership interest in subsidiary |  | 45.99% | 45.99% |
Merger Reserve [Member] |  |  |  |
Statement Line Items [Line Items] |  |  |  |
Other reserves |  | € 247 | € 247 |
Miscellaneous other reserves [member] |  |  |  |
Statement Line Items [Line Items] |  |  |  |
Other reserves |  | 63 | 63 |
Accumulated other comprehensive income [member] |  |  |  |
Statement Line Items [Line Items] |  |  |  |
Other reserves |  | € 90 | 21 |
Treasury Shares [member] |  |  |  |
Statement Line Items [Line Items] |  |  |  |
Dividends proposed or declared before financial statements authorised for issue but not recognised as distribution to owners per share |  | € 0.4 |  |
Dividends paid, ordinary shares per share |  | € 0.80 |  |
Dividends proposed or declared before financial statements authorised for issue but not recognised as distribution to owners |  | € 1,441 |  |
Other Reserves |  |  |  |
Statement Line Items [Line Items] |  |  |  |
Other reserves |  | € 280 | € 211 |

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- Definition A component of equity representing the reserve for the redemption of the entity's own shares. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Name IAS -Number 1 -IssueDate 2017-01-01 -Paragraph 55 -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=1&date=2017-03-01&anchor=para_55&doctype=Standard -URIDate 2017-03-09 + Details
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