Expenses - Restructuring Reserve (Details) - USD ($)	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Restructuring Reserve [Roll Forward]
Beginning balance	$ 201,000,000	$ 56,000,000	$ 56,000,000
Ending balance	175,000,000		201,000,000
Employee Related Costs
Restructuring Reserve [Roll Forward]
Beginning balance	166,000,000	37,000,000	37,000,000
Ending balance	144,000,000		166,000,000
Real Estate Actions
Restructuring Reserve [Roll Forward]
Beginning balance	32,000,000	17,000,000	17,000,000
Ending balance	28,000,000		32,000,000
Asset and Other Write-offs
Restructuring Reserve [Roll Forward]
Beginning balance	3,000,000	2,000,000	2,000,000
Ending balance	3,000,000		3,000,000
State Street Beacon
Restructuring Reserve [Roll Forward]
Accruals	0	$ 17,000,000	245,000,000
Payments and Other Adjustments	(26,000,000)		(100,000,000)
State Street Beacon | Employee Related Costs
Restructuring Reserve [Roll Forward]
Accruals	0		186,000,000
Payments and Other Adjustments	(22,000,000)		(57,000,000)
State Street Beacon | Real Estate Actions
Restructuring Reserve [Roll Forward]
Accruals	0		32,000,000
Payments and Other Adjustments	(4,000,000)		(17,000,000)
State Street Beacon | Asset and Other Write-offs
Restructuring Reserve [Roll Forward]
Accruals	0		27,000,000
Payments and Other Adjustments	$ 0		$ (26,000,000)