Loans and Allowance for Loan Losses - Summary of Troubled Debt Restructurings (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Financing Receivable, Modifications [Line Items]
Total troubled debt restructured loans	$ 608	$ 606
FDIC covered loans [Member]
Financing Receivable, Modifications [Line Items]
Total troubled debt restructured loans	$ 3	$ 4

Recorded Investment in Trouble Debt Restructuring loans on accrual and nonaccrual status.

No definition available.

Name:	ub_TroubleDebtRestructuringLoan
Namespace Prefix:	ub_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant