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Fair Values of Credit Substitutes by Type of Instrument (Detail) ₨ in Millions, $ in Millions | Mar. 31, 2018 INR (₨) | Mar. 31, 2018 USD ($) | Mar. 31, 2017 INR (₨)
--- | --- | --- | ---
Schedule of Available-for-sale Securities [Line Items] |  |  |
Amortized Cost | ₨ 2,229,838.9 | $ 34,247.3 | ₨ 2,072,759.4
Fair Value | 2,222,711.0 | 34,137.8 | 2,111,385.6
Debt securities, other than asset and mortgage-backed securities | Credit Substitutes |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Amortized Cost | 325,159.3 | 4,994.0 | 419,320.5
Fair Value | 324,031.5 | $ 4,976.7 | 419,540.6
Debt securities, other than asset and mortgage-backed securities | Debentures | Credit Substitutes |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Amortized Cost | 290,893.5 |  | 171,180.6
Fair Value | 289,782.9 |  | 171,270.9
Debt securities, other than asset and mortgage-backed securities | Commercial paper | Credit Substitutes |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Amortized Cost | 34,265.8 |  | 248,139.9
Fair Value | ₨ 34,248.6 |  | ₨ 248,269.7

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