Fair Value of Financial Instruments Quantitative Information About Significant Unobservable Inputs (Detail) - Significant Unobservable Inputs (Level 3) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Equity Warrant Asset, Private Portfolio | Other Assets
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value	$ 119,395	$ 128,049
Valuation Technique	Black-Scholes option pricing model	Black-Scholes option pricing model
Volatility	36.70%	36.90%
Risk-Free interest rate	1.80%	1.30%
Marketability discount	16.40%	17.10%
Remaining Life Assumption	45.00%	45.00%
Equity Warrant Asset, Public Portfolio | Other Assets
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value	$ 1,936	$ 764
Valuation Technique	Black-Scholes option pricing model	Black-Scholes option pricing model
Sales Restrictions Discount	15.50%	17.70%
Volatility	47.90%	46.60%
Risk-Free interest rate	2.10%	2.10%
Other venture capital investments | Fair value accounting
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value	$ 919	$ 2,040
Valuation Technique	Private company equity pricing	Private company equity pricing