Risk management - Most important interest rate and credit spread sensitivities at year-end (Detail) - EUR (€) € in Thousands	Dec. 31, 2025	Dec. 31, 2024
Netherlands
Market Risk [Line Items]
Reportable ECL	€ 389,000	€ 270,000
USA
Market Risk [Line Items]
Reportable ECL	105,000	113,000
Germany
Market Risk [Line Items]
Reportable ECL	633,000	548,000
Interest Rate [(BPV) \| Euro \| Trading portfolios, netting
Market Risk [Line Items]
Reportable ECL	(312)	(799)
Interest Rate [(BPV) \| US Dollar \| Trading portfolios, netting
Market Risk [Line Items]
Reportable ECL	(161)	(198)
Interest Rate [(BPV) \| British Pound \| Trading portfolios, netting
Market Risk [Line Items]
Reportable ECL	(133)	(189)
Interest Rate [(BPV) \| Japanese Yen \| Trading portfolios, netting
Market Risk [Line Items]
Reportable ECL	133
Interest Rate [(BPV) \| Korean Won \| Trading portfolios, netting
Market Risk [Line Items]
Reportable ECL		(54)
Interest Rate [(BPV) \| Philippine Peso \| Trading portfolios, netting
Market Risk [Line Items]
Reportable ECL	(98)	(54)
Credit Spread (CSO1) \| Corporate \| Trading portfolios, netting
Market Risk [Line Items]
Reportable ECL	378	162
Credit Spread (CSO1) \| Financial institutions \| Trading portfolios, netting
Market Risk [Line Items]
Reportable ECL	(124)	(489)
Credit Spread (CSO1) \| 1 (AAA) \| Corporate \| Trading portfolios, netting
Market Risk [Line Items]
Reportable ECL	(2)	(2)
Credit Spread (CSO1) \| 1 (AAA) \| Financial institutions \| Trading portfolios, netting
Market Risk [Line Items]
Reportable ECL	(19)	(118)
Credit Spread (CSO1) \| 2–4 (AA) \| Corporate \| Trading portfolios, netting
Market Risk [Line Items]
Reportable ECL	26	(44)
Credit Spread (CSO1) \| 2–4 (AA) \| Financial institutions \| Trading portfolios, netting
Market Risk [Line Items]
Reportable ECL	(13)	(27)
Credit Spread (CSO1) \| 5–7 (A) \| Corporate \| Trading portfolios, netting
Market Risk [Line Items]
Reportable ECL	64	49
Credit Spread (CSO1) \| 5–7 (A) \| Financial institutions \| Trading portfolios, netting
Market Risk [Line Items]
Reportable ECL	(37)	(246)
Credit Spread (CSO1) \| 8–10 (BBB) \| Corporate \| Trading portfolios, netting
Market Risk [Line Items]
Reportable ECL	191	93
Credit Spread (CSO1) \| 8–10 (BBB) \| Financial institutions \| Trading portfolios, netting
Market Risk [Line Items]
Reportable ECL	(54)	(76)
Credit Spread (CSO1) \| 11–13 (BB) \| Corporate \| Trading portfolios, netting
Market Risk [Line Items]
Reportable ECL	38	38
Credit Spread (CSO1) \| 11–13 (BB) \| Financial institutions \| Trading portfolios, netting
Market Risk [Line Items]
Reportable ECL	(14)	(13)
Credit Spread (CSO1) \| 14–16 (B) \| Corporate \| Trading portfolios, netting
Market Risk [Line Items]
Reportable ECL	56	23
Credit Spread (CSO1) \| 14–16 (B) \| Financial institutions \| Trading portfolios, netting
Market Risk [Line Items]
Reportable ECL	8	(12)
Credit Spread (CSO1) \| 17–22 (CCC and NPL) \| Corporate \| Trading portfolios, netting
Market Risk [Line Items]
Reportable ECL	6	4
Credit Spread (CSO1) \| 17–22 (CCC and NPL) \| Financial institutions \| Trading portfolios, netting
Market Risk [Line Items]
Reportable ECL	4	2
Credit Spread (CSO1) \| France \| Trading portfolios, netting
Market Risk [Line Items]
Reportable ECL	178	(113)
Credit Spread (CSO1) \| United Kingdom \| Trading portfolios, netting
Market Risk [Line Items]
Reportable ECL	111
Credit Spread (CSO1) \| Netherlands \| Trading portfolios, netting
Market Risk [Line Items]
Reportable ECL	75	(165)
Credit Spread (CSO1) \| Indonesia \| Trading portfolios, netting
Market Risk [Line Items]
Reportable ECL	(63)
Credit Spread (CSO1) \| South Korea \| Trading portfolios, netting
Market Risk [Line Items]
Reportable ECL	€ 63
Credit Spread (CSO1) \| USA \| Trading portfolios, netting
Market Risk [Line Items]
Reportable ECL		193
Credit Spread (CSO1) \| Poland \| Trading portfolios, netting
Market Risk [Line Items]
Reportable ECL		69
Credit Spread (CSO1) \| Germany \| Trading portfolios, netting
Market Risk [Line Items]
Reportable ECL		€ 49

Netherlands