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Post-Retirement Obligation - Summary of Fair Value of Pension Plan Assets Issued by Sponsor and Subsidiaries (Detail) - BRL (R$)
R$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Disclosure of fair value of plan assets [line items]    
Assets of the pension plan R$ 1,098 R$ 1,114
Non-convertible debentures issued by the Company and subsidiaries [member]    
Disclosure of fair value of plan assets [line items]    
Assets of the pension plan 363 397
Shares issued by the Company [member]    
Disclosure of fair value of plan assets [line items]    
Assets of the pension plan 10 7
Real estate properties of the Foundation, occupied by the Company and subsidiaries [member]    
Disclosure of fair value of plan assets [line items]    
Assets of the pension plan R$ 725 R$ 710