Commitments, Contingencies and Guarantees - Additional Information (Detail) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2020	Dec. 31, 2019	Dec. 31, 2018
Summary Of Commitments And Contingent Liabilities [Line Items]
Lending commitments	$ 159,607	$ 167,244
Notional amount of loan commitments which are protected by SMFG against credit loss	$ 3,210	$ 5,740
Credit loss protection percentage of first loss on loan commitments provided by SMFG	95.00%	95.00%
Approximate amount of maximum protection of first loss on loan commitments provided by SMFG	$ 950	$ 950
SMFG credit loss protection for additional losses percentage	70.00%	70.00%
Maximum protection on additional losses on loan commitments provided by SMFG	$ 750	$ 750
Protection provided by SMFG for additional losses	0	0
Commitments to invest in funds managed by the firm	1,820	2,060
Collateral held by lenders in connection with securities lending indemnifications	21,500	19,140
Lending Commitments Allowance	615	361	$ 286
Amortized cost [Member]
Summary Of Commitments And Contingent Liabilities [Line Items]
Carrying value of lending commitments liabilities	800	527
Lending Commitments Allowance	615	361
Estimated fair value of lending commitments liabilities	4,520	3,050
Amortized cost [Member] | Level 2 [Member]
Summary Of Commitments And Contingent Liabilities [Line Items]
Estimated fair value of lending commitments liabilities	1,420	1,780
Amortized cost [Member] | Level 3 [Member]
Summary Of Commitments And Contingent Liabilities [Line Items]
Estimated fair value of lending commitments liabilities	3,100	1,270
Held for sale [Member]
Summary Of Commitments And Contingent Liabilities [Line Items]
Carrying value of lending commitments liabilities	75	60
Investment Grade Commercial Lending, Relationship Lending Activities [Member]
Summary Of Commitments And Contingent Liabilities [Line Items]
Lending commitments	106,080	102,500
Investment Grade Commercial Lending, Other Investment Banking Activities Member]
Summary Of Commitments And Contingent Liabilities [Line Items]
Lending commitments	14,690	$ 33,470
Residential Mortgage Backed Securities Working Group [Member]
Summary Of Commitments And Contingent Liabilities [Line Items]
Settlement agreement amount	1,800
Litigation settlement liability	1,700
GS Malaysia Development Berhad [Member] | Guarantee Obligations [Member]
Summary Of Commitments And Contingent Liabilities [Line Items]
Agreement in principle to extend a guarantee related to legal and regulatory proceedings	$ 1,400