Risk Management Policy - Comparative Structure of Financial Debt According to Fixed and /or Protected Interest Rate on Gross Debt (Detail)	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Disclosure Of Risk Management Strategy Related To Hedge Accounting [Line Items]
Interest rate risk	92.00%	92.00%
Fixed interest rate [member]
Disclosure Of Risk Management Strategy Related To Hedge Accounting [Line Items]
Interest rate risk	92.00%	92.00%

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