Summary of Significant Accounting Policies, Nature of Operations and Other Developments (Details) (Subsequent event)	Jul. 01, 2014 item
Subsequent event
Summary of significant accounting policies, nature of operations and other developments
Number of employees transferred to the Bank	2,300

Represents the number of employees transferred to the Bank.

No definition available.

Name:	ub_NumberOfEmployeeTransferedToBank
Namespace Prefix:	ub_
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	instant