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REDEEMABLE NONCONTROLLING INTERESTS - NARRATIVE (Details)
1 Months Ended 3 Months Ended 12 Months Ended
Apr. 30, 2019
INR (₨)
Mar. 31, 2020
USD ($)
Mar. 31, 2019
USD ($)
Dec. 31, 2019
INR (₨)
Dec. 31, 2019
USD ($)
Mar. 31, 2020
₨ / shares
Mar. 31, 2020
USD ($)
Dec. 31, 2018
Apr. 21, 2016
Noncontrolling Interest [Line Items]                  
Temporary equity, redemption price per share (in dollars per share) | ₨ / shares           ₨ 216      
Purchase of redeemable noncontrolling interest   $ 524,400,000 $ 425,700,000 ₨ 29,400,000,000 $ 425,700,000        
Redemption consideration value ₨ 24,800,000,000           $ 328,000,000.0    
Percentage of ownership after transaction   92.00%              
ATC, TIPL                  
Noncontrolling Interest [Line Items]                  
Ownership interest percentage       63.00% 63.00%   79.00%    
Noncontrolling interest, ownership percentage by noncontrolling owners (as a percent)             21.00% 37.00%  
Eure-et-Loir                  
Noncontrolling Interest [Line Items]                  
Ownership interest percentage             51.00%    
Noncontrolling interest, ownership percentage by noncontrolling owners (as a percent)             49.00%    
Redeemable noncontrolling interest carrying amount             $ 2,700,000    
Ghana and Uganda Subsidiaries                  
Noncontrolling Interest [Line Items]                  
Purchase of redeemable noncontrolling interest   $ 524,400,000              
Ownership interest percentage       51.00% 51.00%   100.00%    
Payments for repurchase of redeemable noncontrolling interest, interest portion   $ 1,400,000              
Tata Teleservices Limited                  
Noncontrolling Interest [Line Items]                  
Percentage of ownership before transaction 100.00%                
Tata Teleservices Limited | ATC, TIPL                  
Noncontrolling Interest [Line Items]                  
Percentage of ownership before transaction       50.00% 50.00%        
IDFC | ATC, TIPL                  
Noncontrolling Interest [Line Items]                  
Percentage of ownership before transaction       100.00% 100.00%        
Viom Transaction                  
Noncontrolling Interest [Line Items]                  
Percentage of interests acquired                 51.00%