Unsecured Borrowings - Additional Information (Detail) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019	Dec. 31, 2004
Debt Instrument [Line Items]
Subordinated debt maturities, range, end	Dec. 31, 2067
Minimum redemption or purchase price required	$ 253.0
Debt Instrument, Maturity Date Range, End	Dec. 31, 2067
Subordinated Debt [Member]
Debt Instrument [Line Items]
Subordinated debt maturities, range, start	Dec. 31, 2021	Dec. 31, 2021
Subordinated debt maturities, range, end	Dec. 31, 2045	Dec. 31, 2045
Trust Preferred Securities purchased, par amount	$ 8.1
Trust Preferred Securities purchased, carrying value	$ 12.5
Debt Instrument, Maturity Date Range, End	Dec. 31, 2045	Dec. 31, 2045
Goldman Sachs Capital I [Member]
Debt Instrument [Line Items]
Junior subordinated debentures issued to Goldman Sachs Capital I (Trust)			$ 2,840.0
Guaranteed preferred beneficial interests issued to third parties			2,750.0
Common beneficial interests issued to Group Inc.			$ 85.0
Junior subordinated debt, outstanding par amount	$ 968.0	$ 976.0
Trust Preferred Securities, outstanding par amount	939.0	947.0
Common beneficial interests, outstanding par amount	29.0	$ 29.0
Trust Preferred Securities purchased, par amount	7.9
Trust Preferred Securities purchased, carrying value	11.0
Common beneficial interests delivered to the Trust	$ 0.2
Interest Rate of Junior Subordinated Debentures held by certain third parties	6.345%
Maturity date of Junior Subordinated Debentures held by certain third parties	Feb. 15, 2034
Interest Rate of Junior Subordinated Debentures issued to Trust, Fixed	6.345%
Maturity date of Junior Subordinated Debentures issued to Trust	Feb. 15, 2034