Fair Value of Financial Instruments - Quantitative Information About Significant Unobservable Inputs (Detail) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2018	Dec. 31, 2017	Mar. 31, 2018	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair value	$ 138,754	$ 122,250	$ 132,507	$ 130,849	$ 124,239	$ 130,853
Level 3 | Unconsolidated venture capital and private equity fund investments | Fair value accounting
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Valuation Technique	Private company equity pricing
Level 3 | Equity warrant asset, public portfolio | Other assets
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair value	$ 2,065	$ 1,936
Valuation Technique	Black-Scholes option pricing model	Black-Scholes option pricing model
Sales restrictions discount rate	18.60%	15.50%
Volatility	48.60%	47.90%
Risk-Free interest rate	2.80%	2.10%
Level 3 | Equity warrant assets, private portfolio | Other assets
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair value	$ 135,688	$ 119,395
Valuation Technique	Black-Scholes option pricing model	Black-Scholes option pricing model
Volatility	37.80%	36.70%
Risk-Free interest rate	2.50%	1.80%
Marketability discount	16.60%	16.40%
Remaining life assumption	45.00%	45.00%
Unconsolidated venture capital and private equity fund investments | Level 3 | Fair value accounting
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair value	$ 1,001	$ 919
Valuation Technique		Private company equity pricing