Line of Business Information - Components of Other (Details) - USD ($) $ in Millions	3 Months Ended			6 Months Ended
	Jun. 30, 2026	Mar. 31, 2026	Jun. 30, 2025	Jun. 30, 2026	Jun. 30, 2025
Segment Reporting [Line Items]
Foreign exchange trading services	$ 494		$ 393	$ 929	$ 730
Repositioning charges		$ (89)
Client rescoping (expense impact)		89
Income before income tax expense	1,389		889	2,358	1,711
Income decrease due to client rescoping				42
Income Statement Location [Axis]: us-gaap:CommunicationsAndInformationTechnology
Segment Reporting [Line Items]
Client rescoping (expense impact)		9
Income decrease due to client rescoping				18
Income Statement Location [Axis]: us-gaap:LaborAndRelatedExpense
Segment Reporting [Line Items]
Client rescoping (expense impact)		79			100
Income Statement Location [Axis]: us-gaap:OccupancyNet
Segment Reporting [Line Items]
Client rescoping (expense impact)		1
Income Statement Location [Axis]: stt:FeesAndCommissionsFrontOfficeSoftwareAndData
Segment Reporting [Line Items]
Income decrease due to client rescoping				24
Other
Segment Reporting [Line Items]
Foreign exchange trading services	0		3	0	3
Client rescoping (revenue impact)	0		(24)	0	(24)
Repositioning charges	0		(100)	(89)	(100)
Client rescoping (expense impact)	0	$ 41	18	41	18
Other	0		1	0	1
Income before income tax expense	$ 0		$ (138)	$ (130)	$ (138)