Commercial Paper and Other Short-Term Borrowings - Narrative (Details) ¥ in Billions	12 Months Ended
	Dec. 31, 2017 USD ($)	Dec. 31, 2017 JPY (¥)	Dec. 31, 2016 USD ($)
Commercial paper
Standby Letters of Credit
Weighted average remaining maturity days	73 days
MUB
Standby Letters of Credit
Weighted average remaining maturity days	55 days
Affiliated Entity
Standby Letters of Credit
Weighted average remaining maturity days	259 days
Affiliated Entity | MUAH
Standby Letters of Credit
Amount drawn under facility	$ 801,000,000	¥ 90	$ 692,000,000
Line of Credit | MUAH
Standby Letters of Credit
Revolving credit facility borrowing capacity	$ 1,400,000,000.0
Maturity extension period	100 days