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27. REVENUE (Details 3) - BRL (R$)
R$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Revenue [abstract]      
Construction and upgrades revenue R$ 201 R$ 312 R$ 138
Construction and upgrades costs (147) (220) (96)
Margin R$ 54 R$ 92 R$ 42
Mark-up (%) 36.73% 41.82% 43.75%
Operation and maintenance revenue R$ 279 R$ 352 R$ 343
Operation and maintenance cost (223) (388) (215)
Margin R$ 56 R$ (36) R$ 128
Mark-up (%) 25.11% (9.28%) 59.53%