Investments, AFS debt securities - Portfolio of AFS Debt Securities (Detail) ₨ in Millions, $ in Millions	Mar. 31, 2025 INR (₨)	Mar. 31, 2025 USD ($)	Mar. 31, 2024 INR (₨)
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	₨ 9,714,673.5	$ 113,715.0	₨ 8,306,160.5
Gross Unrealized Gains	209,610.0	2,453.6	65,491.0
Gross Unrealized Losses	14,109.2	165.2	76,164.4
Fair Value	9,910,174.3	116,003.4	8,295,487.1
Debt securities, other than asset and mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	9,602,070.0		8,167,306.7
Gross Unrealized Gains	208,495.5		65,153.1
Gross Unrealized Losses	14,041.3		75,520.9
Fair Value	9,796,524.2	$ 114,673.1	8,156,938.9
Debt securities, other than asset and mortgage-backed securities \| Government of India securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	6,871,303.8		6,105,313.9
Gross Unrealized Gains	135,750.3		41,090.5
Gross Unrealized Losses	6,077.2		52,213.3
Fair Value	7,000,976.9		6,094,191.1
Debt securities, other than asset and mortgage-backed securities \| State government securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,646,894.8		1,188,315.0
Gross Unrealized Gains	39,861.0		7,600.4
Gross Unrealized Losses	2,498.1		12,234.4
Fair Value	1,684,257.7		1,183,681.0
Debt securities, other than asset and mortgage-backed securities \| Government securities outside India
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	10,280.7		3,556.3
Gross Unrealized Gains	74.5		3.9
Gross Unrealized Losses	0.3		18.4
Fair Value	10,354.9		3,541.8
Debt securities, other than asset and mortgage-backed securities \| Credit substitutes
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	161,308.5		133,197.8
Gross Unrealized Gains	473.8		321.7
Gross Unrealized Losses	602.7		2,032.6
Fair Value	161,179.6		131,486.9
Debt securities, other than asset and mortgage-backed securities \| Other corporate/financial institution bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	912,282.2		736,923.7
Gross Unrealized Gains	32,335.9		16,136.6
Gross Unrealized Losses	4,863.0		9,022.2
Fair Value	939,755.1		744,038.1
Mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	59.4		78.1
Gross Unrealized Gains	2.3		2.3
Gross Unrealized Losses	0.8		1.2
Fair Value	60.9		79.2
Asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	112,544.1		138,775.7
Gross Unrealized Gains	1,112.2		335.6
Gross Unrealized Losses	67.1		642.3
Fair Value	113,589.2		138,469.0
Securities with gross unrealized losses
Schedule of Available-for-sale Securities [Line Items]
Fair Value	1,022,535.5		3,706,475.3
Securities with gross unrealized gains
Schedule of Available-for-sale Securities [Line Items]
Fair Value	₨ 8,887,638.8		₨ 4,589,011.8

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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