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Financial Risk Management - Summary of Effect of Foreign Currency Derivatives Impacts and Commercial Transactions Exposures (Detail) - EUR (€) € in Millions | 3 Months Ended | 6 Months Ended
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Jun. 30, 2018 | Jun. 30, 2017 | Jun. 30, 2018 | Jun. 30, 2017
Disclosure of nature and extent of risks arising from financial instruments [line items] |  |  |  |
Realized gains / (losses) on foreign currency derivatives - net | € (1) | € (1) | € 1 | € (11)
Included in Unrealized (losses) / gains on foreign currency derivatives - net | 4 | 7 | 3 | 13
Derivatives that do not qualify for hedge accounting [member] | Other gains and losses [member] |  |  |  |
Disclosure of nature and extent of risks arising from financial instruments [line items] |  |  |  |
Realized gains / (losses) on foreign currency derivatives - net | (1) | (2) | (2) | (11)
Included in Unrealized (losses) / gains on foreign currency derivatives - net | 5 | 5 | 4 | 12
Derivatives that qualify for hedge accounting [member] | Other gains and losses [member] |  |  |  |
Disclosure of nature and extent of risks arising from financial instruments [line items] |  |  |  |
Realized gains / (losses) on foreign currency derivatives - net | (2) |  | (1) |
Realized gains / (losses) in ineffective portion of derivatives | 0 | 0 | 0 | 0
Derivatives that qualify for hedge accounting [member] | Revenue [member] |  |  |  |
Disclosure of nature and extent of risks arising from financial instruments [line items] |  |  |  |
Realized gains / (losses) on foreign currency derivatives - net | 2 | (1) | 4 | (2)
Included in Unrealized (losses) / gains on foreign currency derivatives - net | (1) | 2 | (1) | 1
Derivatives that qualify for hedge accounting [member] | Other comprehensive income (loss) [member] |  |  |  |
Disclosure of nature and extent of risks arising from financial instruments [line items] |  |  |  |
Included in Unrealized (losses) / gains on foreign currency derivatives - net | (22) | 25 | (11) | 28
(Losses) / gains reclassified from cash flow hedge reserve to Consolidated Income Statement | € (1) | € (1) | € (3) | € 1

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