Expenses - Narrative (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 Cost and Reserve [Line Items]
Restructuring charges	$ 33			$ 10			$ 112	$ 120
State Street Beacon
Restructuring Cost and Reserve [Line Items]
Restructuring charges	33	$ 62	$ 16	10	$ 13	$ 97
State Street Beacon | Severance and Benefit Costs
Restructuring Cost and Reserve [Line Items]
Restructuring charges	$ 23	$ 60	$ 14	$ 8	$ (1)	$ 86