Quantitative Information About Significant Unobservable Inputs (Detail) (USD $) In Thousands, unless otherwise specified	6 Months Ended	9 Months Ended
	Jun. 30, 2012	Sep. 30, 2012		Dec. 31, 2011	Sep. 30, 2011		Jun. 30, 2011	Dec. 31, 2010
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value	$ 828,326	$ 841,722		$ 799,962	$ 753,677		$ 679,960	$ 547,608
Other venture capital investments | Fair value accounting
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value		118,622
Valuation Technique		Private company equity pricing
Equity Warrant Asset | Other assets
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value	$ 68,619	$ 64,691	[1]	$ 63,030	$ 54,418	[1]	$ 49,777	$ 43,537
Valuation Technique		Modifed Black-Scholes option pricing model
Volatility		49.80%
Risk-Free interest rate		0.30%
Marketability discount	15.00%	22.50%
Remaining life assumption	40.00%	45.00%

[1] Realized and unrealized gains (losses) are recorded on the line item “gains on derivative instruments, net”, a component of noninterest income.