Commercial Paper and Other Short-Term Borrowings (Details) $ in Millions, ¥ in Billions	6 Months Ended
	Jun. 30, 2019 USD ($)	Jun. 30, 2019 JPY (¥)	Dec. 31, 2018 USD ($)
Short-term Debt [Line Items]
Total commercial paper and other short-term borrowings	$ 8,685		$ 9,263
MUFG Union Bank N.A
Short-term Debt [Line Items]
Total commercial paper and other short-term borrowings	$ 7,838		$ 8,182
MUFG Union Bank N.A | Commercial paper
Short-term Debt [Line Items]
Weighted average interest rate at period end	2.44%	2.44%	2.39%
Commercial paper	$ 888		$ 382
MUFG Union Bank N.A | Federal Home Loan Bank Advances
Short-term Debt [Line Items]
Weighted average interest rate at period end	2.56%	2.56%	2.52%
Federal home loan bank advances	$ 6,950		$ 7,800
MUFG Americas Holding Corporation
Short-term Debt [Line Items]
Weighted average interest rate at period end	2.73%	2.73%	3.01%
Total commercial paper and other short-term borrowings	$ 847		$ 1,081
Short-term debt due to parent	$ 69		$ 145
MUFG Americas Holding Corporation | Affiliated Entity
Short-term Debt [Line Items]
Weighted average interest rate at period end	(0.09%)	(0.09%)	(0.07%)
Short term debt due to affiliates	$ 778	¥ 84	$ 936
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