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Loans and Borrowings - Summary of Long - Term Loans and Borrowings (Detail)
€ in Millions, ₨ in Millions, ر.س in Millions, £ in Millions, R$ in Millions, $ in Millions, $ in Millions, $ in Millions
 12 Months Ended
Mar. 31, 2018
INR (₨)
Mar. 31, 2018
USD ($)
Mar. 31, 2018
AUD ($)
Mar. 31, 2018
CAD ($)
Mar. 31, 2018
BRL (R$)
Mar. 31, 2017
INR (₨)
Mar. 31, 2017
EUR (€)
Mar. 31, 2017
USD ($)
Mar. 31, 2017
AUD ($)
Mar. 31, 2017
GBP (£)
Mar. 31, 2017
SAR (ر.س)
Mar. 31, 2017
CAD ($)
Disclosure of detailed information about borrowings [line items]                        
Unsecured loans ₨ 54,688         ₨ 17,391            
Obligations under finance leases 5,442         8,280            
Liabilities directly associated with assets held for sale (1,469)                      
Finance lease liabilities 3,973         8,280            
Total 58,661         25,671            
Non-current portion of long term loans and borrowings 45,268 $ 695       19,611            
Current portion of long term loans and borrowings 13,393         6,060            
Total 58,661         25,671            
Unsecured external commercial borrowing 1.81% June 2018 [member] | U.S.Dollar [member]                        
Disclosure of detailed information about borrowings [line items]                        
Unsecured loans ₨ 9,777 $ 150       9,728   $ 150        
Interest rate 1.94% 1.94% 1.94% 1.94% 1.94%              
Final maturity June 2018                      
Unsecured term loan [member] | U.S.Dollar [member]                        
Disclosure of detailed information about borrowings [line items]                        
Unsecured loans ₨ 40,715 $ 625       118   $ 2        
Final maturity June 2021                      
Unsecured term loan [member] | U.S.Dollar [member] | Bottom of range [member]                        
Disclosure of detailed information about borrowings [line items]                        
Interest rate 1.90% 1.90% 1.90% 1.90% 1.90%              
Unsecured term loan [member] | U.S.Dollar [member] | Top of range [member]                        
Disclosure of detailed information about borrowings [line items]                        
Interest rate 3.81% 3.81% 3.81% 3.81% 3.81%              
Unsecured term loan [member] | Canadian Dollar [member]                        
Disclosure of detailed information about borrowings [line items]                        
Unsecured loans ₨ 3,660     $ 72   4,131           $ 85
Final maturity July 2021                      
Unsecured term loan [member] | Canadian Dollar [member] | Bottom of range [member]                        
Disclosure of detailed information about borrowings [line items]                        
Interest rate 1.20% 1.20% 1.20% 1.20% 1.20%              
Unsecured term loan [member] | Canadian Dollar [member] | Top of range [member]                        
Disclosure of detailed information about borrowings [line items]                        
Interest rate 3.26% 3.26% 3.26% 3.26% 3.26%              
Unsecured term loan [member] | Indian Rupee [member]                        
Disclosure of detailed information about borrowings [line items]                        
Unsecured loans ₨ 366         714            
Final maturity December 2021                      
Unsecured term loan [member] | Indian Rupee [member] | Bottom of range [member]                        
Disclosure of detailed information about borrowings [line items]                        
Interest rate 8.30% 8.30% 8.30% 8.30% 8.30%              
Unsecured term loan [member] | Indian Rupee [member] | Top of range [member]                        
Disclosure of detailed information about borrowings [line items]                        
Interest rate 9.40% 9.40% 9.40% 9.40% 9.40%              
Unsecured term loan [member] | Australian Dollar [member]                        
Disclosure of detailed information about borrowings [line items]                        
Unsecured loans ₨ 92   $ 2     116     $ 2      
Interest rate 4.65% 4.65% 4.65% 4.65% 4.65%              
Final maturity January 2022                      
Unsecured term loan [member] | Great British Pound [member]                        
Disclosure of detailed information about borrowings [line items]                        
Unsecured loans ₨ 42         73       £ 1    
Interest rate 2.93% 2.93% 2.93% 2.93% 2.93%              
Final maturity February 2022                      
Unsecured term loan [member] | EURO [member]                        
Disclosure of detailed information about borrowings [line items]                        
Unsecured loans ₨ 24         1,282 € 19          
Interest rate 2.98% 2.98% 2.98% 2.98% 2.98%              
Final maturity December 2020                      
Unsecured term loan [member] | Brazil, Brazil Real                        
Disclosure of detailed information about borrowings [line items]                        
Unsecured loans ₨ 12       R$ 1              
Interest rate 14.04% 14.04% 14.04% 14.04% 14.04%              
Final maturity May 2019                      
Unsecured term loan [member] | Saudi Arabian Riyal [member]                        
Disclosure of detailed information about borrowings [line items]                        
Unsecured loans           ₨ 1,229         ر.س 71