Fair Value of Financial Instruments - Quantitative Information About Significant Unobservable Inputs (Detail) - Level 3 $ in Thousands	Sep. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)
Unconsolidated venture capital and private equity fund investments
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Fair value	$ 1,001	$ 919
Equity warrant asset, public portfolio
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Fair value	$ 1,805	$ 1,936
Equity warrant asset, public portfolio | Volatility
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Weighted Average	0.531	0.479
Equity warrant asset, public portfolio | Risk-Free interest rate
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Weighted Average	0.030	0.021
Equity warrant asset, public portfolio | Sales restrictions discount
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Weighted Average	0.141	0.155
Equity warrant assets, private portfolio
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Fair value	$ 138,684	$ 119,395
Equity warrant assets, private portfolio | Volatility
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Weighted Average	0.382	0.367
Equity warrant assets, private portfolio | Risk-Free interest rate
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Weighted Average	0.028	0.018
Equity warrant assets, private portfolio | Marketability discount
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Weighted Average	0.172	0.164
Equity warrant assets, private portfolio | Remaining life assumption
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Weighted Average	0.450	0.450