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Financial Instruments - Summary of Significant Unobservable Inputs to Valuation (Detail) - Unquoted equity investments [member] - INR (₨)
₨ in Millions
12 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Discounted cash flow model [member] | Long term growth rate [member]    
Disclosure of significant unobservable inputs used in fair value measurement of assets and liabilities [line items]    
Valuation technique Discounted cash flow model  
Significant unobservable input Long term growth rate  
Movement by 0.50%  
Increase ₨ 201  
Decrease ₨ (187)  
Discounted cash flow model [member] | Discount rate [member]    
Disclosure of significant unobservable inputs used in fair value measurement of assets and liabilities [line items]    
Significant unobservable input Discount rate  
Movement by 0.50%  
Increase ₨ (243)  
Decrease ₨ 256  
Third party quote [member] | Revenue achievement [member]    
Disclosure of significant unobservable inputs used in fair value measurement of assets and liabilities [line items]    
Valuation technique   Third party quote
Significant unobservable input   Revenue achievement
Movement by   1.00%
Increase   ₨ 18
Decrease   ₨ (18)