Commitments, Contingencies and Guarantees - Additional Information (Detail) - USD ($)	3 Months Ended
	Mar. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Summary Of Commitments And Contingent Liabilities [Line Items]
Lending commitments	$ 151,965,000,000	$ 167,244,000,000
Notional amount of loan commitments which are protected by SMFG against credit loss	$ 4,050,000,000.00	$ 5,740,000,000
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	$ 0
Commitments to invest in funds managed by the firm	2,000,000,000.00	2,060,000,000.00
Collateral held by lenders in connection with securities lending indemnifications	17,560,000,000	19,140,000,000
Lending Commitments Allowance	335,000,000	361,000,000	$ 286,000,000
Amortized cost [Member]
Summary Of Commitments And Contingent Liabilities [Line Items]
Carrying value of lending commitments liabilities	460,000,000	527,000,000
Lending Commitments Allowance	335,000,000	361,000,000
Estimated fair value of lending commitments liabilities	5,890,000,000	3,050,000,000.00
Amortized cost [Member] | Level 2 [Member]
Summary Of Commitments And Contingent Liabilities [Line Items]
Estimated fair value of lending commitments liabilities	3,290,000,000	1,780,000,000
Amortized cost [Member] | Level 3 [Member]
Summary Of Commitments And Contingent Liabilities [Line Items]
Estimated fair value of lending commitments liabilities	2,600,000,000	1,270,000,000
Held for sale [Member]
Summary Of Commitments And Contingent Liabilities [Line Items]
Carrying value of lending commitments liabilities	196,000,000	60,000,000
Investment Grade Commercial Lending, Relationship Lending Activities [Member]
Summary Of Commitments And Contingent Liabilities [Line Items]
Lending commitments	88,610,000,000	102,500,000,000
Investment Grade Commercial Lending, Other Investment Banking Activities Member]
Summary Of Commitments And Contingent Liabilities [Line Items]
Lending commitments	26,690,000,000	$ 33,470,000,000
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	$ 1,550,000,000