Investments, AFS debt securities - Portfolio of AFS Debt Securities (Detail) ₨ in Millions, $ in Millions	Mar. 31, 2026 INR (₨)	Mar. 31, 2026 USD ($)	Mar. 31, 2025 INR (₨)
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	₨ 10,459,069.8	$ 111,468.3	₨ 9,714,673.5
Gross Unrealized Gains	40,574.6	432.4	209,610.0
Gross Unrealized Losses	252,143.6	2,687.2	14,109.2
Fair Value	10,247,500.8	109,213.5	9,910,174.3
Debt securities, other than asset and mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	10,346,275.0		9,602,070.0
Gross Unrealized Gains	40,039.1		208,495.5
Gross Unrealized Losses	252,030.0		14,041.3
Fair Value	10,134,284.1	$ 108,006.9	9,796,524.2
Debt securities, other than asset and mortgage-backed securities \| Government of India securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	7,003,249.8		6,871,303.8
Gross Unrealized Gains	32,044.5		135,750.3
Gross Unrealized Losses	192,178.3		6,077.2
Fair Value	6,843,116.0		7,000,976.9
Debt securities, other than asset and mortgage-backed securities \| State government securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,347,446.1		1,646,894.8
Gross Unrealized Gains	2,531.4		39,861.0
Gross Unrealized Losses	53,772.1		2,498.1
Fair Value	2,296,205.4		1,684,257.7
Debt securities, other than asset and mortgage-backed securities \| Government securities outside India
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	191,002.2		10,280.7
Gross Unrealized Gains	73.9		74.5
Gross Unrealized Losses	4.5		0.3
Fair Value	191,071.6		10,354.9
Debt securities, other than asset and mortgage-backed securities \| Credit substitutes
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	273,691.4		161,308.5
Gross Unrealized Gains	907.3		473.8
Gross Unrealized Losses	1,575.6		602.7
Fair Value	273,023.1		161,179.6
Debt securities, other than asset and mortgage-backed securities \| Other corporate/financial institution bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	530,885.5		912,282.2
Gross Unrealized Gains	4,482.0		32,335.9
Gross Unrealized Losses	4,499.5		4,863.0
Fair Value	530,868.0		939,755.1
Mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	44.2		59.4
Gross Unrealized Gains	0.9		2.3
Gross Unrealized Losses	0.2		0.8
Fair Value	44.9		60.9
Asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	112,750.6		112,544.1
Gross Unrealized Gains	534.6		1,112.2
Gross Unrealized Losses	113.4		67.1
Fair Value	113,171.8		113,589.2
Securities with gross unrealized losses
Schedule of Available-for-sale Securities [Line Items]
Fair Value	6,706,415.2		1,022,535.5
Securities with gross unrealized gains
Schedule of Available-for-sale Securities [Line Items]
Fair Value	₨ 3,541,085.6		₨ 8,887,638.8

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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