Related Party Transactions (Details)		1 Months Ended	12 Months Ended
		Sep. 30, 2008 USD ($)	Dec. 31, 2015 USD ($)		Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Jul. 01, 2014 USD ($) employee
Related Party Transaction [Line Items]
Available-for-sale Securities			$ 14,344,000,000		$ 13,724,000,000
Additional paid in capital	[1]				31,000,000	$ 1,200,000,000
Increase in retained earnings			4,000,000	[1]	(5,000,000)	(9,000,000)
Liabilities			100,546,000,000		98,471,000,000
Salaries and benefits expense			2,248,000,000		1,785,000,000	1,631,000,000
Other expense			3,438,000,000		2,823,000,000	2,713,000,000
Other assets			5,377,000,000		4,630,000,000
Other liabilities			2,023,000,000		1,897,000,000
Noninterest income			1,530,000,000		1,123,000,000	876,000,000
Affiliated Entity | Senior and Subordinated Debt
Related Party Transaction [Line Items]
Due to Related Parties, Noncurrent			4,600,000,000
BTMU
Related Party Transaction [Line Items]
Other expense			21,000,000		9,000,000	13,000,000
Unsecured revolving credit facility with BTMU		$ 500,000,000
Unsecured revolving credit facility years		3 years
Unsecured revolving credit facility amount outstanding			0
Project Related expenditure			11,000,000		15,000,000	19,000,000
Derivative notional amount			669,000,000		634,000,000
Net unrealized losses on derivatives			7,000,000		1,000,000
Noninterest income			44,000,000		38,000,000	$ 33,000,000
Amount of overdraft balance outstanding			0
BTMU | Affiliated Entity
Related Party Transaction [Line Items]
Available-for-sale Securities							$ 70,000,000
Employee-related liabilities							30,000,000
Additional paid in capital					31,000,000
Liabilities							$ 9,000,000
BTMU | United States | Affiliated Entity
Related Party Transaction [Line Items]
Number of U.S. employees | employee							2,300
Retained Earnings
Related Party Transaction [Line Items]
Increase in retained earnings	[1]		0		$ (9,000,000)
BTMU | Affiliated Entity
Related Party Transaction [Line Items]
Fees from affiliates			747,000,000
Other expense			21,000,000
Other assets			71,000,000
Other liabilities			23,000,000
MUAH | Affiliated Entity | Senior and Subordinated Debt
Related Party Transaction [Line Items]
Due to Related Parties, Noncurrent			300,000,000
MUB | Affiliated Entity | Senior and Subordinated Debt
Related Party Transaction [Line Items]
Due to Related Parties, Noncurrent			4,300,000,000
Support Services | BTMU | Affiliated Entity
Related Party Transaction [Line Items]
Fees from affiliates			546,000,000
Salaries and benefits expense			$ 507,000,000

[1] For additional information on the capital contribution, refer to Note 20 to these consolidated financial statements.