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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, 2020
Mar. 31, 2019
Disclosure of significant unobservable inputs used in fair value measurement of assets and liabilities [line items]    
Valuation technique   Discounted cash flow model
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 Long term growth rate
Movement by 0.50% 0.50%
Increase ₨ 298 ₨ 201
Decrease ₨ (273) ₨ (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 Discount rate
Movement by 0.50% 0.50%
Increase ₨ (388) ₨ (243)
Decrease ₨ 404 ₨ 256