Commitments, Contingencies and Guarantees - Additional Information (Detail) - USD ($)	9 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
Summary Of Commitments And Contingent Liabilities [Line Items]
Lending commitments	$ 158,319,000,000	$ 137,582,000,000
Notional amount of loan commitments which are protected by SMFG against credit loss	$ 7,690,000,000	$ 15,520,000,000
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	$ 950,000,000	$ 950,000,000
SMFG credit loss protection for additional losses percentage	70.00%	70.00%
Maximum protection on additional losses on loan commitments provided by SMFG	$ 750,000,000	$ 750,000,000
Protection provided by SMFG for additional losses	0	550,000,000
Commitments to invest in funds managed by the firm	2,070,000,000.00	2,420,000,000
Collateral held by lenders in connection with securities lending indemnifications	19,800,000,000	28,750,000,000
Investment Grade Commercial Lending, Relationship Lending Activities [Member]
Summary Of Commitments And Contingent Liabilities [Line Items]
Lending commitments	99,900,000,000	93,990,000,000
Investment Grade Commercial Lending, Other Investment Banking Activities Member]
Summary Of Commitments And Contingent Liabilities [Line Items]
Lending commitments	30,080,000,000.00	$ 27,920,000,000
Residential Mortgage Backed Securities Working Group [Member]
Summary Of Commitments And Contingent Liabilities [Line Items]
Settlement agreement amount	1,800,000,000
Litigation settlement liability	$ 1,450,000,000