Major Components of Investment Securities Portfolio (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012		Dec. 31, 2011
Investment Holdings [Line Items]
Amortized Cost	$ 10,835,536		$ 10,384,741
Unrealized Gains	213,230		157,105
Unrealized Losses	(1,036)		(5,800)
Carrying Value	11,047,730		10,536,046
Non-marketable securities	1,163,815		1,004,440
Investment securities	12,211,545		11,540,486
Fair value accounting | Venture capital and private equity fund investments
Investment Holdings [Line Items]
Non-marketable securities	658,409	[1]	611,824	[1]
Fair value accounting | Other venture capital investments
Investment Holdings [Line Items]
Non-marketable securities	118,622	[2]	124,121	[2]
Fair value accounting | Other investments
Investment Holdings [Line Items]
Non-marketable securities	0		987
Equity method accounting | Other investments
Investment Holdings [Line Items]
Non-marketable securities	141,761	[3]	68,252	[3]
Equity method accounting | Low income housing tax credit funds
Investment Holdings [Line Items]
Non-marketable securities	66,806		34,894
Cost method accounting | Venture capital and private equity fund investments
Investment Holdings [Line Items]
Non-marketable securities	158,275	[4]	145,007	[4]
Cost method accounting | Other investments
Investment Holdings [Line Items]
Non-marketable securities	19,942		19,355
U.S. treasury securities
Investment Holdings [Line Items]
Amortized Cost	25,101		25,233
Unrealized Gains	328		731
Unrealized Losses	0		0
Carrying Value	25,429		25,964
U.S. agency debentures
Investment Holdings [Line Items]
Amortized Cost	2,857,336		2,822,158
Unrealized Gains	78,814		52,864
Unrealized Losses	0		(90)
Carrying Value	2,936,150		2,874,932
Residential mortgage-backed securities | Mortgage-backed Securities, Issued by US Government Sponsored Enterprises
Investment Holdings [Line Items]
Amortized Cost	1,535,934		1,529,466
Unrealized Gains	54,648		34,926
Unrealized Losses	0		(106)
Carrying Value	1,590,582		1,564,286
Residential mortgage-backed securities | Agency-issued collateralized mortgage obligations | Fixed rate
Investment Holdings [Line Items]
Amortized Cost	4,119,707		3,317,285
Unrealized Gains	49,160		56,546
Unrealized Losses	(742)		(71)
Carrying Value	4,168,125		3,373,760
Residential mortgage-backed securities | Agency-issued collateralized mortgage obligations | Variable rate
Investment Holdings [Line Items]
Amortized Cost	1,928,853		2,416,158
Unrealized Gains	10,951		1,554
Unrealized Losses	(5)		(4,334)
Carrying Value	1,939,799		2,413,378
Agency-issued commercial mortgage-backed securities
Investment Holdings [Line Items]
Amortized Cost	273,021		176,646
Unrealized Gains	6,486		2,047
Unrealized Losses	0		0
Carrying Value	279,507		178,693
Municipal bonds and notes
Investment Holdings [Line Items]
Amortized Cost	91,643		92,241
Unrealized Gains	8,578		8,257
Unrealized Losses	0		0
Carrying Value	100,221		100,498
Equity securities
Investment Holdings [Line Items]
Amortized Cost	3,941		5,554
Unrealized Gains	4,265		180
Unrealized Losses	(289)		(1,199)
Carrying Value	$ 7,917		$ 4,535

[1] The following table shows the amounts of venture capital and private equity fund investments held by the following consolidated funds and our ownership percentage of each fund at September 30, 2012 and December 31, 2011 (fair value accounting):

[2] The following table shows the amounts of other venture capital investments held by the following consolidated funds and our ownership percentage of each fund at September 30, 2012 and December 31, 2011 (fair value accounting): September 30, 2012 December 31, 2011(Dollars in thousands) Amount Ownership % Amount Ownership %Silicon Valley BancVentures, LP $16,737 10.7% $17,878 10.7%SVB Capital Partners II, LP (i) 55,686 5.1 61,099 5.1SVB India Capital Partners I, LP 42,713 14.4 42,832 14.4SVB Capital Shanghai Yangpu Venture Capital Fund 3,486 6.8 2,312 6.8Total other venture capital investments $118,622 $124,121

[3] The following table shows the carrying value and our ownership percentage of each investment at September 30, 2012 and December 31, 2011 (equity method accounting): September 30, 2012 December 31, 2011(Dollars in thousands) Amount Ownership % Amount Ownership %Gold Hill Venture Lending 03, LP (i) $9,187 9.3% $16,072 9.3%Gold Hill Capital 2008, LP (ii) 20,491 15.5 19,328 15.5Partners for Growth II, LP 3,199 24.2 3,785 24.2China Joint Venture investment (iii) 78,484 50.0 — —Other investments 30,400 N/A 29,067 N/A Total other investments (equity method accounting) $141,761 $68,252

[4] Represents investments in 325 and 329 funds (primarily venture capital funds) at September 30, 2012 and December 31, 2011, respectively, where our ownership interest is less than 5% of the voting interests of each such fund and in which we do not have the ability to exercise significant influence over the partnerships operating activities and financial policies. For the three months ended September 30, 2012, we recognized OTTI losses of $0.4 million resulting from other-than-temporary declines in value for 17 of the 325 investments. For the nine months ended September 30, 2012, we recognized OTTI losses of $0.9 million resulting from other-than-temporary declines in value for 43 of the 325 investments. The OTTI losses are included in net gains on investment securities, a component of noninterest income. We concluded that any declines in value for the remaining investments were temporary and as such, no OTTI was required to be recognized. At September 30, 2012, the carrying value of these venture capital and private equity fund investments (cost method accounting) was $158.3 million, and the estimated fair value was $190.0 million.