Fair Value of Financial Instruments - Quantitative Information About Significant Unobservable Inputs (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015		Mar. 31, 2014		Dec. 31, 2014		Dec. 31, 2013
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair value	$ 1,396,414		1,455,357		$ 1,316,784		$ 1,314,951
Equity warrant assets | Other assets
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair value	122,261	[1]	87,642	[1]	114,698	[1]	99,891	[1]
Level 3 | Other venture capital investments | Fair value accounting
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair value	78,850				71,204
Valuation Technique	Private company equity pricing		Private company equity pricing
Level 3 | Equity warrant assets | Lower Limit
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Sales restrictions discount	10.00%
Level 3 | Equity warrant assets | Upper Limit
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Sales restrictions discount	20.00%
Level 3 | Equity warrant assets | Investments
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair value	1,472				1,681
Level 3 | Equity warrant assets | Other assets
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Valuation Technique	Modified Black-Scholes option pricing model		Modified Black-Scholes option pricing model
Volatility	46.60%		42.60%
Risk-Free interest rate	1.80%		1.70%
Sales restrictions discount	15.20%	[2]	17.80%	[2]
Level 3 | Equity Warrant Asset, Private Portfolio | Investments
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair value	$ 120,789				$ 113,017
Level 3 | Equity Warrant Asset, Private Portfolio | Other assets
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Valuation Technique	Modified Black-Scholes option pricing model		Modified Black-Scholes option pricing model
Volatility	38.10%		38.30%
Risk-Free interest rate	0.70%		0.90%
Marketability discount	18.10%	[3]	20.00%	[3]
Remaining life assumption	45.00%	[4]	45.00%	[4]

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

[2] We adjust quoted market prices of public companies, which are subject to certain sales restrictions. Sales restriction discounts generally range from 10 percent to 20 percent depending on the duration of the sales restrictions, which typically range from 3 to 6 months.

[3] Our marketability discount is applied to all private company warrants to account for a general lack of liquidity due to the private nature of the associated underlying company. The quantitative measure used is based upon various option-pricing models. On a quarterly basis, a sensitivity analysis is performed on our marketability discount.

[4] We adjust the contractual remaining term of private company warrants based on our estimate of the actual remaining life, which we determine by utilizing historical data on cancellations and exercises. At March 31, 2015, the weighted average contractual remaining term was 2.61 years, compared to our estimated remaining life of 5.80 years. On a quarterly basis, a sensitivity analysis is performed on our remaining life assumption.