Long-Term Debt - Summary of Long Term Debt (Parenthetical) (Detail)	Jun. 30, 2012	Sep. 30, 2012 UnionBanCal Corporation [Member] Fixed rate 3.50% notes due June 2022 [Member] Senior debt [Member]	Dec. 31, 2011 UnionBanCal Corporation [Member] Fixed rate 3.50% notes due June 2022 [Member] Senior debt [Member]	Sep. 30, 2012 UnionBanCal Corporation [Member] Fixed rate 5.25% notes due December 2013 [Member] Subordinated debt [Member]	Dec. 31, 2011 UnionBanCal Corporation [Member] Fixed rate 5.25% notes due December 2013 [Member] Subordinated debt [Member]	Sep. 30, 2012 Subsidiaries [Member] Fixed and floating rate Federal Home Loan Bank advances with maturities ranging from January 2013 to February 2016 [Member] Senior debt [Member]	Dec. 31, 2011 Subsidiaries [Member] Fixed and floating rate Federal Home Loan Bank advances with maturities ranging from January 2013 to February 2016 [Member] Senior debt [Member]	Sep. 30, 2012 Subsidiaries [Member] Floating rate notes due March 2012 [Member] Senior debt [Member]	Dec. 31, 2011 Subsidiaries [Member] Floating rate notes due March 2012 [Member] Senior debt [Member]	Sep. 30, 2012 Subsidiaries [Member] Floating rate notes due June 2014 [Member] Senior debt [Member]	Dec. 31, 2011 Subsidiaries [Member] Floating rate notes due June 2014 [Member] Senior debt [Member]	Sep. 30, 2012 Subsidiaries [Member] Fixed rate 2.125% notes due December 2013 [Member] Senior debt [Member]	Dec. 31, 2011 Subsidiaries [Member] Fixed rate 2.125% notes due December 2013 [Member] Senior debt [Member]	Sep. 30, 2012 Subsidiaries [Member] Fixed rate 3.00% notes due June 2016 [Member] Senior debt [Member]	Dec. 31, 2011 Subsidiaries [Member] Fixed rate 3.00% notes due June 2016 [Member] Senior debt [Member]	Sep. 30, 2012 Subsidiaries [Member] Fixed rate 2.125% notes due June 2017 [Member] Senior debt [Member]	Dec. 31, 2011 Subsidiaries [Member] Fixed rate 2.125% notes due June 2017 [Member] Senior debt [Member]	Sep. 30, 2012 Subsidiaries [Member] Fixed rate 6.03% notes due July 2014 (related to consolidated VIE) [Member] Note Payable [Member]	Dec. 31, 2011 Subsidiaries [Member] Fixed rate 6.03% notes due July 2014 (related to consolidated VIE) [Member] Note Payable [Member]	Sep. 30, 2012 Subsidiaries [Member] Fixed rate 5.95% notes due May 2016 [Member] Subordinated debt [Member]	Dec. 31, 2011 Subsidiaries [Member] Fixed rate 5.95% notes due May 2016 [Member] Subordinated debt [Member]
Debt Instrument [Line Items]
Debt instrument, interest rate	99.699%	3.50%	3.50%	5.25%	5.25%	1.45%	1.75%	0.20%	0.76%	1.36%	1.48%	2.125%	2.125%	3.00%	3.00%	2.125%	2.125%	6.03%	6.03%	5.95%	5.95%