Expenses - Restructuring Reserve (Details) - USD ($) $ in Millions	3 Months Ended
	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020
Restructuring Reserve [Roll Forward]
Beginning balance	$ 152	$ 186	$ 196	$ 138	$ 164	$ 198
Accruals			2
Payments and Other Adjustments	(23)	(34)	(11)	(10)	(26)	(34)
Ending balance	129	152	186	128	138	164
State Street Beacon
Restructuring Reserve [Roll Forward]
Accruals			(1)
Employee Related Costs
Restructuring Reserve [Roll Forward]
Beginning balance	146	180	190	132	157	190
Accruals			0
Payments and Other Adjustments	(23)	(34)	(9)	(10)	(25)	(33)
Ending balance	123	146	180	122	132	157
Employee Related Costs | State Street Beacon
Restructuring Reserve [Roll Forward]
Accruals			(1)
Real Estate Actions
Restructuring Reserve [Roll Forward]
Beginning balance	6	6	6	5	6	7
Accruals			2
Payments and Other Adjustments	0	0	(2)	1	(1)	(1)
Ending balance	6	6	6	6	5	6
Real Estate Actions | State Street Beacon
Restructuring Reserve [Roll Forward]
Accruals			0
Asset and Other Write-offs
Restructuring Reserve [Roll Forward]
Beginning balance	0	0	0	1	1	1
Accruals			0
Payments and Other Adjustments	0	0	0	(1)	0	0
Ending balance	$ 0	$ 0	0	$ 0	$ 1	$ 1
Asset and Other Write-offs | State Street Beacon
Restructuring Reserve [Roll Forward]
Accruals			$ 0