Deposits - Types and Sources of the Firm's Deposits (Parenthetical) (Detail) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2020 USD ($) Arrangements	Dec. 31, 2019 USD ($)
Deposits [Abstract]
Deposits at fair value	$ 18,401	$ 17,765
Weighted average maturity of time deposits	1 year 3 months 18 days	1 year 8 months 12 days
Number of deposit sweep program contractual arrangements | Arrangements	12
Deposits insured by the FDIC	$ 126,880	$ 103,980
Deposits insured by the U.K.'s Financial Services Compensation Scheme	$ 24,990	$ 15,860