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Significant Accounting Policies - Additional Information (Detail)
₨ / shares in Units, ₨ in Millions, $ in Millions
 12 Months Ended
Apr. 01, 2019
INR (₨)
Mar. 31, 2020
INR (₨)
Vote
₨ / shares
shares
Mar. 31, 2019
INR (₨)
₨ / shares
shares
Mar. 31, 2020
USD ($)
shares
Mar. 08, 2019
₨ / shares
Mar. 31, 2018
INR (₨)
₨ / shares
shares
Jun. 15, 2017
₨ / shares
Disclosure of classes of share capital [line items]              
Authorized share capital   ₨ 25,274          
Par value per share | ₨ / shares   ₨ 2 ₨ 2   ₨ 2 ₨ 2 ₨ 2
Number of vote | Vote   1          
Undistributed earnings after taxes not freely available for distribution- Capital Reserve   ₨ 1,139 ₨ 1,139     ₨ 1,139  
Capital redemption reserve   646 0     767  
Lease liabilities ₨ 15,379 19,198          
Right of use asset ₨ 13,630 16,748   $ 222      
Weighted average discount rate on lease liabilities 5.70%            
Reclassified from Property Plant and Equipment to right-of-use assets ₨ 1,243            
Reclassification of finance lease to lease liabilities 2,002            
Finance lease liability current 1,506            
Finance lease liability noncurrent 496            
Reclassification of prepaid rent on leasehold land and other assets to right of use assets 2,222            
Depreciation expense from right-of-use assets   5,911          
Interest expenses on lease liabilities   914          
Expense relating to low value assets   44          
Expense relating to short value assets   2,085          
Leases not yet commenced   1,399          
IFRS Sixteen [Member]              
Disclosure of classes of share capital [line items]              
Increase decrease in retained earnings 872            
Net deferred tax assets 138            
Gross carrying amount [member]              
Disclosure of classes of share capital [line items]              
Right of use asset   22,689 17,095        
Reclassified from Property Plant and Equipment to right-of-use assets 3,420            
Accumulated depreciation and amortisation [member]              
Disclosure of classes of share capital [line items]              
Right of use asset   5,941          
Reclassified from Property Plant and Equipment to right-of-use assets ₨ 2,177            
Depreciation expense from right-of-use assets   5,911          
Increase (decrease) due to application of IFRS 15 [member]              
Disclosure of classes of share capital [line items]              
Expected reduction in opening retained earnings on adoption of IFRS 15     2,279        
Special economic zone re-investment reserve [member]              
Disclosure of classes of share capital [line items]              
Undistributed earnings after taxes not freely available for distribution- Capital Reserve   ₨ 43,804 ₨ 28,565     ₨ 0  
Bottom of range [member]              
Disclosure of classes of share capital [line items]              
Perentage of voting power of another entity   20.00%          
Bottom of range [member] | IFRS Sixteen [Member]              
Disclosure of classes of share capital [line items]              
Term of lease not yet commenced   2 years          
Top of range [member]              
Disclosure of classes of share capital [line items]              
Perentage of voting power of another entity   50.00%          
Top of range [member] | IFRS Sixteen [Member]              
Disclosure of classes of share capital [line items]              
Term of lease not yet commenced   8 years          
Treasury shares [member]              
Disclosure of classes of share capital [line items]              
Treasury shares held | shares   22,746,081 27,353,853 22,746,081   23,097,216  
Ordinary Shares [member]              
Disclosure of classes of share capital [line items]              
Authorized shares | shares   12,504,500,000   12,504,500,000      
Par value per share | ₨ / shares   ₨ 2          
Preference shares [member]              
Disclosure of classes of share capital [line items]              
Authorized shares | shares   25,000,000   25,000,000      
Par value per share | ₨ / shares   ₨ 10          
10% Optionally Convertible Cumulative Preference Shares [member]              
Disclosure of classes of share capital [line items]              
Authorized shares | shares   150,000   150,000      
Par value per share | ₨ / shares   ₨ 100