Business Segments (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011	Dec. 31, 2011
Results of operations - Market View
Net interest income (expense)	$ 659	$ 614	$ 1,312	$ 1,232
Noninterest income (expense)	175	240	377	480
Noninterest expense (income)	599	578	1,213	1,193
Income tax expense (benefit)	67	131	118	245
Net income (loss)	182	239	373	470
Deduct: Net loss from noncontrolling interests	5	3	9	7
Net Income attributable to UnionBanCal Corporation (UNBC)	187	242	382	477
Total assets, end of period - Market View	87,939		87,939		89,676
Retail Banking [Member]
Results of operations - Market View
Net interest income (expense)	272	276	540	543
Noninterest income (expense)	58	71
Noninterest income (expense)			114	136
Total revenue	330	347	654	679
Noninterest expense (income)	263	274
Noninterest expense (income)			527	542
Credit expense (income)	6	7	12	13
Income (loss) before income taxes and including noncontrolling interests	61	66	115	124
Income tax expense (benefit)	24	26	45	49
Net income (loss)	37	40	70	75
Deduct: Net loss from noncontrolling interests	0		0
Net Income attributable to UnionBanCal Corporation (UNBC)	37	40	70	75
Total assets, end of period - Market View	26,448	24,478	26,448	24,478
Corporate Banking [Member]
Results of operations - Market View
Net interest income (expense)	337	314	672	629
Noninterest income (expense)	138	153
Noninterest income (expense)			275	279
Total revenue	475	467	947	908
Noninterest expense (income)	243	243
Noninterest expense (income)			483	473
Credit expense (income)	38	49	77	101
Income (loss) before income taxes and including noncontrolling interests	194	175	387	334
Income tax expense (benefit)	50	43	102	81
Net income (loss)	144	132	285	253
Deduct: Net loss from noncontrolling interests	0		0
Net Income attributable to UnionBanCal Corporation (UNBC)	144	132	285	253
Total assets, end of period - Market View	36,378	30,778	36,378	30,778
All Other Segments [Member]
Results of operations - Market View
Net interest income (expense)	71	44	141	97
Noninterest income (expense)	(6)	36
Noninterest income (expense)			18	101
Total revenue	65	80	159	198
Noninterest expense (income)	104	76
Noninterest expense (income)			226	205
Credit expense (income)	(58)	(150)	(104)	(310)
Income (loss) before income taxes and including noncontrolling interests	19	154	37	303
Income tax expense (benefit)	2	72	(11)	133
Net income (loss)	17	82	48	170
Deduct: Net loss from noncontrolling interests	5	3	9	7
Net Income attributable to UnionBanCal Corporation (UNBC)	22	85	57	177
Total assets, end of period - Market View	27,356	26,968	27,356	26,968
Reconciling Items [Member]
Results of operations - Market View
Net interest income (expense)	(20)	(20)	(40)	(37)
Noninterest income (expense)	(16)	(20)
Noninterest income (expense)			(31)	(36)
Total revenue	(36)	(40)	(71)	(73)
Noninterest expense (income)	(12)	(15)
Noninterest expense (income)			(24)	(27)
Credit expense (income)	0		0
Income (loss) before income taxes and including noncontrolling interests	(24)	(25)	(47)	(46)
Income tax expense (benefit)	(10)	(10)	(19)	(18)
Net income (loss)	(14)	(15)	(28)	(28)
Deduct: Net loss from noncontrolling interests	0		0
Net Income attributable to UnionBanCal Corporation (UNBC)	(14)	(15)	(28)	(28)
Total assets, end of period - Market View	(2,243)	(2,131)	(2,243)	(2,131)
UnionBanCal Corporation [Member]
Results of operations - Market View
Net interest income (expense)	659	614	1,312	1,232
Noninterest income (expense)	175	240	377	480
Total revenue	834	854	1,689	1,712
Noninterest expense (income)	599	578	1,213	1,193
Credit expense (income)	(14)	(94)	(15)	(196)
Income (loss) before income taxes and including noncontrolling interests	249	370	491	715
Income tax expense (benefit)	67	131	118	245
Net income (loss)	182	239	373	470
Deduct: Net loss from noncontrolling interests	5	3	9	7
Net Income attributable to UnionBanCal Corporation (UNBC)	187	242	382	477
Total assets, end of period - Market View	$ 87,939	$ 80,093	$ 87,939	$ 80,093