Commercial Paper and Other Short-Term Borrowings - Narrative (Details) ¥ in Billions	1 Months Ended	12 Months Ended
	Sep. 30, 2008 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2016 JPY (¥)	Dec. 31, 2015 USD ($)
Federal funds purchased and securities sold under repurchase agreements
Standby Letters of Credit
Weighted average remaining maturity days		3 days
Commercial Paper
Standby Letters of Credit
Weighted average remaining maturity days		70 days
BTMU
Standby Letters of Credit
Revolving credit facility borrowing capacity	$ 500,000,000
Unsecured revolving credit facility years	3 years
MUB
Standby Letters of Credit
Weighted average remaining maturity days		45 days
Affiliated Entity
Standby Letters of Credit
Weighted average remaining maturity days		33 days
Affiliated Entity | MUAH
Standby Letters of Credit
Amount drawn under facility		$ 692,000,000	¥ 81	$ 898,000,000
Line of Credit | MUAH
Standby Letters of Credit
Revolving credit facility borrowing capacity		$ 1,400,000,000.0
Maturity extension period		100 days