Expenses - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Restructuring Cost and Reserve [Line Items]
Net repositioning charges	$ 133	$ 110	$ 300
Reduction of real estate footprint	13.00%
Charles River Development
Restructuring Cost and Reserve [Line Items]
Acquisitions costs	$ 54	79	31
Compensation and Employee Benefits
Restructuring Cost and Reserve [Line Items]
Net repositioning charges	82	98	259
Occupancy Costs
Restructuring Cost and Reserve [Line Items]
Net repositioning charges	$ 51	$ 12	$ 41