Derivative Financial Instruments - Impact of Derivatives on Consolidated Statement of Income (Details) - Derivatives not designated as hedging instruments - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2026	Jun. 30, 2025	Jun. 30, 2026	Jun. 30, 2025
Derivative [Line Items]
Amount of gain (loss) on derivative recognized in income	$ 348	$ 301	$ 695	$ 594
Foreign exchange contracts \| Income Statement Location [Axis]: us-gaap:InterestExpenseOperating
Derivative [Line Items]
Amount of gain (loss) on derivative recognized in income	4	42	33	125
Foreign exchange contracts \| Income Statement Location [Axis]: stt:FeesAndCommissionsForeignExchangeTradingServices
Derivative [Line Items]
Amount of gain (loss) on derivative recognized in income	376	279	699	512
Interest rate contracts
Derivative [Line Items]
Amount of gain (loss) on derivative recognized in income	4	6	5	12
Other derivative contracts \| Income Statement Location [Axis]: us-gaap:LaborAndRelatedExpense
Derivative [Line Items]
Amount of gain (loss) on derivative recognized in income	(12)	(16)	(26)	(51)
Other derivative contracts \| Income Statement Location [Axis]: us-gaap:NoninterestIncomeOther
Derivative [Line Items]
Amount of gain (loss) on derivative recognized in income	$ (24)	$ (10)	$ (16)	$ (4)