Fair Value of Financial Instruments - Quantitative Information About Significant Unobservable Inputs (Detail) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016	Jun. 30, 2017	Sep. 30, 2016	Jun. 30, 2016	Dec. 31, 2015
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair value	$ 140,972	$ 130,853	$ 130,849	$ 145,260	$ 129,851	$ 137,208
Level 3 | Other venture capital investments | Fair value accounting
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair value	$ 1,897	$ 2,040
Valuation Technique	Private company equity pricing	Private company equity pricing
Level 3 | Equity warrant asset, public portfolio | Other assets
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair value	$ 21,660	$ 764
Valuation Technique	Black-Scholes option pricing model	Black-Scholes option pricing model
Sales restrictions discount rate	19.90%	17.70%
Volatility	37.50%	46.60%
Risk-Free interest rate	1.80%	2.10%
Level 3 | Equity warrant assets, private portfolio | Other assets
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair value	$ 117,415	$ 128,049
Valuation Technique	Black-Scholes option pricing model	Black-Scholes option pricing model
Volatility	36.70%	36.90%
Risk-Free interest rate	1.50%	1.30%
Marketability discount	16.60%	17.10%
Remaining life assumption	45.00%	45.00%