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Post-Retirement Obligations - Disclosure of Changes in the Fair Value of Plan Assets (Detail) - Pension plans and retirement supplement plans [member] - BRL (R$)
R$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Disclosure of fair value of plan assets [line items]      
Beginning balance R$ 8,128 R$ 6,703 R$ 8,051
Real return on the investments 1,100 2,105 (730)
Contributions from the employer 159 127 116
Benefits paid (841) (807) (734)
Ending balance R$ 8,546 R$ 8,128 R$ 6,703