Commercial Paper and Other Short-Term Borrowings - Narrative (Details) $ in Millions, ¥ in Billions	12 Months Ended
	Dec. 31, 2018 USD ($)	Dec. 31, 2018 JPY (¥)	Dec. 31, 2017 USD ($)
Federal Home Loan Bank Borrowings [Member]
Standby Letters of Credit
Weighted average remaining maturity days	168 days
Commercial paper
Standby Letters of Credit
Weighted average remaining maturity days	97 days
MUB
Standby Letters of Credit
Weighted average remaining maturity days	53 days
Affiliated Entity
Standby Letters of Credit
Weighted average remaining maturity days	244 days
Affiliated Entity | MUAH
Standby Letters of Credit
Amount drawn under facility	$ 936	¥ 103	$ 801
Unsecured Debt [Member] | MUSA and Various other Non-Bank Subsidiaries
Standby Letters of Credit
Revolving credit facility borrowing capacity	1,400
Line of Credit | MUAH
Standby Letters of Credit
Revolving credit facility borrowing capacity	$ 1,400	¥ 160
Maturity extension period	100 days