Commercial Paper and Other Short-Term Borrowings - Summary of Commercial Paper and Other Short-Term Borrowings (Details) $ in Millions, ¥ in Billions	Dec. 31, 2016 USD ($)	Dec. 31, 2016 JPY (¥)	Dec. 31, 2015 USD ($)
Commercial Paper and Short-Term Borrowings
Total commercial paper and other short-term borrowings	$ 2,360		$ 3,425
MUB
Commercial Paper and Short-Term Borrowings
Total commercial paper and other short-term borrowings	989		1,002
MUAH
Commercial Paper and Short-Term Borrowings
Short-term debt due to BTMU, with weighted average interest rates of 0.49% and 0.64% at December 31, 2016 and December 31, 2015, respectively	679		1,525
Total commercial paper and other short-term borrowings	$ 1,371		$ 2,423
Weighted average interest rate	0.49%	0.49%	0.64%
Federal Funds Purchased [Member] | MUB
Commercial Paper and Short-Term Borrowings
Federal funds purchased, with weighted average interest rates of 0.50% and 0.11% at December 31, 2016 and December 31, 2015, respectively	$ 26		$ 8
Weighted average interest rate	0.50%	0.50%	0.11%
Commercial Paper | MUB
Commercial Paper and Short-Term Borrowings
Commercial paper, with a weighted average interest rate of 0.55% and 0.24% at December 31, 2016 and December 31, 2015, respectively	$ 263		$ 994
Weighted average interest rate	0.55%	0.55%	0.24%
Federal Home Loan Bank advances | MUB
Commercial Paper and Short-Term Borrowings
Federal Home Loan Bank advances, with a weighted average interest rate of 0.59% at December 31, 2016	$ 700		$ 0
Weighted average interest rate	0.59%	0.59%
Affiliated Entity | MUAH
Commercial Paper and Short-Term Borrowings
Short-term debt due to affiliates, with weighted average interest rates of (0.04)% and 0.28% at December 31, 2016 and December 31, 2015, respectively	$ 692	¥ 81	$ 898
Weighted average interest rate	(0.04%)	(0.04%)	0.28%