Fair Value of Financial Instruments - Quantitative Information About Significant Unobservable Inputs (Detail) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Jun. 30, 2015	Dec. 31, 2014	Jun. 30, 2014	Dec. 31, 2013
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair value	$ 131,833	$ 137,260	$ 123,427	$ 117,989	$ 136,706	$ 451,979
Equity warrant assets | Other assets
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair value	128,443	$ 93,397	$ 120,037		$ 87,151
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
Level 3 | Equity warrant assets | Investments
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair value	$ 4,795			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
Volatility	39.40%	42.60%
Risk-Free interest rate	1.50%	1.70%
Sales restrictions discount	14.20%	17.80%
Level 3 | Equity Warrant Asset, Private Portfolio | Investments
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair value	$ 123,648			$ 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
Volatility	37.10%	38.30%
Risk-Free interest rate	0.70%	0.90%
Marketability discount	17.30%	20.00%
Remaining life assumption	45.00%	45.00%