Commercial Paper and Other Short-Term Borrowings (Details) $ in Millions, ¥ in Billions	9 Months Ended
	Sep. 30, 2020 USD ($)	Sep. 30, 2020 JPY (¥)	Dec. 31, 2019 USD ($)
Short-term Debt [Line Items]
Total commercial paper and other short-term borrowings	$ 849		$ 6,484
MUFG Union Bank N.A
Short-term Debt [Line Items]
Total commercial paper and other short-term borrowings	$ 764		$ 6,158
MUFG Union Bank N.A | Commercial paper
Short-term Debt [Line Items]
Weighted average interest rate at period end	0.73%	0.73%	1.98%
Commercial paper	$ 514		$ 58
MUFG Union Bank N.A | Federal Home Loan Bank Advances
Short-term Debt [Line Items]
Weighted average interest rate at period end	0.21%	0.21%	1.86%
Federal home loan bank advances	$ 250		$ 6,100
MUFG Americas Holding Corporation
Short-term Debt [Line Items]
Weighted average interest rate at period end	0.57%	0.57%	2.27%
Total commercial paper and other short-term borrowings	$ 85		$ 326
Short-term debt due to parent	$ 9		$ 69
MUFG Americas Holding Corporation | Affiliated Entity
Short-term Debt [Line Items]
Weighted average interest rate at period end	(0.08%)	(0.08%)	(0.10%)
Short term debt due to affiliates	$ 76	¥ 8	$ 257
Line of Credit | MUFG Americas Holding Corporation
Short-term Debt [Line Items]
Maximum borrowing capacity	$ 1,500	¥ 160
Line of credit facility, extension period (in days)	100 days