Credit Concentrations - Credit Concentration, Government and Federal Agency Obligations (Detail) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
U.S. Government and Agency Obligations Held By The Firm [Member]
Concentration Risk [Line Items]
Concentration risk, Credit risk, Financial instrument, Maximum exposure	$ 180,041	$ 167,097
Concentration risk, Credit risk, Financial instrument, Maximum exposure, As a percentage of total Assets	15.90%	16.80%
Non-U.S. Government and Agency Obligations Held By The Firm [Member]
Concentration Risk [Line Items]
Concentration risk, Credit risk, Financial instrument, Maximum exposure	$ 71,140	$ 44,875
Concentration risk, Credit risk, Financial instrument, Maximum exposure, As a percentage of total Assets	6.30%	4.50%