Fair Value of Financial Instruments - Quantitative Information About Significant Unobservable Inputs (Detail) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2016	Dec. 31, 2017	Mar. 31, 2017
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair value	$ 132,507	$ 130,853	$ 122,250	$ 124,239
Level 3 | Unconsolidated venture capital and private equity fund investments (1) | 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	$ 1,641		1,936
Valuation Technique	Black-Scholes option pricing model	Black-Scholes option pricing model
Sales restrictions discount rate	13.70%	15.50%
Volatility	46.30%	47.90%
Risk-Free interest rate	2.70%	2.10%
Level 3 | Equity warrant assets, private portfolio | Other assets
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair value	$ 129,865		119,395
Valuation Technique	Black-Scholes option pricing model	Black-Scholes option pricing model
Volatility	37.00%	36.70%
Risk-Free interest rate	2.20%	1.80%
Marketability discount	16.40%	16.40%
Remaining life assumption	45.00%	45.00%
Unconsolidated venture capital and private equity fund investments (1) | Level 3 | Fair value accounting
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair value	$ 1,001		$ 919
Valuation Technique		Private company equity pricing