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Borrowings (Amounts at Risk) (Details) - Nomura Holdings Inc. - USD ($)
$ in Thousands
3 Months Ended 12 Months Ended
Mar. 31, 2024
Dec. 31, 2023
Borrowings [Line Items]    
Repurchase Agreement Counterparty, Amount at Risk $ 260,829 $ 216,886
Repurchase Agreement Counterparty, Weighted Average Maturity of Agreements 164 days 246 days
Fair Value, Concentration of Risk, Maximum Amount of Loss    
Borrowings [Line Items]    
Percentage of Total Outstanding Unpaid Principal Balance 16.80% 14.20%