Fair Value of Financial Instruments - Quantitative Information About Significant Unobservable Inputs (Detail) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014	Dec. 31, 2013
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair value	$ 123,427	$ 136,706	$ 125,651	$ 117,989	$ 478,435	$ 451,979
Equity warrant assets | Other assets
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair value	120,037	$ 87,151	$ 122,261		$ 87,642
Level 3 | Other venture capital investments | Fair value accounting
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair value	$ 3,390			71,204
Valuation Technique	Private company equity pricing	Private company equity pricing
Level 3 | Equity warrant assets | Investments
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair value	$ 4,440			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	38.40%	42.60%
Risk-Free interest rate	1.90%	1.70%
Sales restrictions discount	18.50%	17.80%
Level 3 | Equity Warrant Asset, Private Portfolio | Investments
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair value	$ 115,597			$ 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	37.90%	38.30%
Risk-Free interest rate	0.80%	0.90%
Marketability discount	17.80%	20.00%
Remaining life assumption	45.00%	45.00%