Loans - Summary of Concentration of Corporate Loans (Detail) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Product Information [Line Items]
Loans and Leases Receivable, Gross	$ 115,557	$ 110,345
Concentration Risk,Percentage	100.00%	100.00%
Corporate Loans [Member]
Product Information [Line Items]
Loans and Leases Receivable, Gross	$ 51,465	$ 46,307
Consumer Retail [Member]
Product Information [Line Items]
Concentration Risk,Percentage	7.00%	7.00%
Diversified Industrials [Member]
Product Information [Line Items]
Concentration Risk,Percentage	22.00%	17.00%
Financial Institutions [Member]
Product Information [Line Items]
Concentration Risk,Percentage	10.00%	10.00%
Funds [Member]
Product Information [Line Items]
Concentration Risk,Percentage	9.00%	9.00%
Health Care [Member]
Product Information [Line Items]
Concentration Risk,Percentage	7.00%	8.00%
Natural Resources & Utilities [Member]
Product Information [Line Items]
Concentration Risk,Percentage	12.00%	12.00%
Real Estate [Member]
Product Information [Line Items]
Concentration Risk,Percentage	7.00%	7.00%
Technology, Media & Telecommunications [Member]
Product Information [Line Items]
Concentration Risk,Percentage	17.00%	17.00%
Other [Member]
Product Information [Line Items]
Concentration Risk,Percentage	9.00%	13.00%