Long-Term Debt - Summary of Long-Term Debt (Detail) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Debt Instrument [Line Items]
Long-term debt	$ 6,058	$ 5,622
Subsidiaries [Member]
Debt Instrument [Line Items]
Long-term debt	5,259	4,819
UnionBanCal
Debt Instrument [Line Items]
Long-term debt	799	803
Senior debt [Member] | Fixed rate 3.50% notes due June 2022 [Member] | UnionBanCal
Debt Instrument [Line Items]
Long-term debt	397	397
Senior debt [Member] | Federal Home Loan Bank advances [Member] | Subsidiaries [Member]
Debt Instrument [Line Items]
Long-term debt	1,800	2,100
Senior debt [Member] | Floating rate notes due June 2014 [Member] | Subsidiaries [Member]
Debt Instrument [Line Items]
Long-term debt	300	300
Senior debt [Member] | Fixed rate 2.125% notes due December 2013 [Member] | Subsidiaries [Member]
Debt Instrument [Line Items]
Long-term debt	400	399
Senior debt [Member] | Fixed rate 3.00% notes due June 2016 [Member] | Subsidiaries [Member]
Debt Instrument [Line Items]
Long-term debt	699	699
Senior debt [Member] | Fixed rate 2.125% notes due June 2017 [Member] | Subsidiaries [Member]
Debt Instrument [Line Items]
Long-term debt	499	499
Subordinated Debt [Member] | Fixed rate 5.25% notes due December 2013 [Member] | UnionBanCal
Debt Instrument [Line Items]
Long-term debt	402	406
Subordinated Debt [Member] | Fixed rate 5.95% notes due May 2016 [Member] | Subsidiaries [Member]
Debt Instrument [Line Items]
Long-term debt	724	729
Subordinated Debt [Member] | Floating rate notes due June 2023 | BTMU
Debt Instrument [Line Items]
Long-term debt	750
Note payable [Member] | Fixed rate 6.03% notes due July 2014 (related to consolidated VIE) [Member] | Subsidiaries [Member]
Debt Instrument [Line Items]
Long-term debt	6	6
Junior subordinated debt payable to trusts [Member] | Floating rate [Member] | Subsidiaries [Member]
Debt Instrument [Line Items]
Long-term debt	66	66
Junior subordinated debt payable to trusts [Member] | Capital Lease Obligations [Member] | Subsidiaries [Member]
Debt Instrument [Line Items]
Long-term debt	$ 15	$ 21