Commercial Paper and Other Short-Term Borrowings - Narrative (Details) $ in Millions, ¥ in Billions	12 Months Ended
	Dec. 31, 2019 USD ($)	Dec. 31, 2019 JPY (¥)	Dec. 31, 2018 USD ($)
Federal Home Loan Bank Borrowings [Member]
Standby Letters of Credit
Weighted average remaining maturity days	160 days
Commercial paper
Standby Letters of Credit
Weighted average remaining maturity days	45 days
MUFG Union Bank N.A [Member]
Standby Letters of Credit
Weighted average remaining maturity days	55 days
Affiliated Entity
Standby Letters of Credit
Weighted average remaining maturity days	141 days
Affiliated Entity | MUAH
Standby Letters of Credit
Amount drawn under facility	$ 257	¥ 28	$ 936
Unsecured Debt [Member] | MUSA and Various other Non-Bank Subsidiaries
Standby Letters of Credit
Revolving credit facility borrowing capacity	1,500
Line of Credit | MUAH
Standby Letters of Credit
Revolving credit facility borrowing capacity	$ 1,500	¥ 160
Maturity extension period	100 days