Fair Value of Financial Instruments - Quantitative Information About Significant Unobservable Inputs (Detail) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2016	Dec. 31, 2015	Jun. 30, 2016	Sep. 30, 2015	Jun. 30, 2015	Dec. 31, 2014
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair value	$ 145,260	$ 137,208	$ 129,851	$ 131,833	$ 123,427	$ 117,989
Level 3 | Other venture capital investments | Fair value accounting
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair value	$ 2,040	$ 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	$ 19,367	$ 1,786
Valuation Technique	Modified Black-Scholes option pricing model	Modified Black-Scholes option pricing model
Sales restrictions discount rate	11.70%	18.00%
Volatility	36.10%	38.10%
Risk-Free interest rate	1.20%	2.10%
Level 3 | Equity warrant assets, private portfolio | Other assets
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair value	$ 123,853	$ 133,382
Valuation Technique	Modified Black-Scholes option pricing model	Modified Black-Scholes option pricing model
Volatility	36.60%	36.00%
Risk-Free interest rate	0.80%	1.10%
Marketability discount	17.50%	16.60%
Remaining life assumption	45.00%	45.00%