Expenses - Restructuring Reserve (Details) - USD ($) $ in Millions	3 Months Ended						9 Months Ended
	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Sep. 30, 2017	Sep. 30, 2016
Restructuring Reserve [Roll Forward]
Accruals for Beacon	$ 33			$ 10			$ 112	$ 120
State Street Beacon
Restructuring Reserve [Roll Forward]
Beginning balance	100	$ 54	$ 56	82	$ 108	$ 23	56	23
Accruals for Beacon	33	62	16	10	13	97
Payments and Other Adjustments	(16)	(16)	(18)	(18)	(39)	(12)
Ending balance	117	100	54	74	82	108	117	74
State Street Beacon | Employee Related Costs
Restructuring Reserve [Roll Forward]
Beginning balance	87	38	37	55	91	9	37	9
Accruals for Beacon	23	60	14	8	(1)	86
Payments and Other Adjustments	(10)	(11)	(13)	(14)	(35)	(4)
Ending balance	100	87	38	49	55	91	100	49
State Street Beacon | Real Estate Actions
Restructuring Reserve [Roll Forward]
Beginning balance	11	14	17	22	10	11	17	11
Accruals for Beacon	9	0	0	3	15	0
Payments and Other Adjustments	(5)	(3)	(3)	(3)	(3)	(1)
Ending balance	15	11	14	22	22	10	15	22
State Street Beacon | Asset and Other Write-offs
Restructuring Reserve [Roll Forward]
Beginning balance	2	2	2	5	7	3	2	3
Accruals for Beacon	1	2	2	(1)	(1)	11
Payments and Other Adjustments	(1)	(2)	(2)	(1)	(1)	(7)
Ending balance	$ 2	$ 2	$ 2	$ 3	$ 5	$ 7	$ 2	$ 3