Investment Securities - Summary of Venture Capital and Private Equity Fund Investments Held by Consolidated Funds and Percentage Ownership (Detail) - Non-marketable securities - Fair value accounting - Consolidated venture capital and private equity fund investments - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Investment Holdings [Line Items]
Amount	$ 133,007	$ 128,111
Strategic Investors Fund, LP
Investment Holdings [Line Items]
Amount	$ 13,972	$ 14,673
Percentage of ownership	12.60%	12.60%
Capital Preferred Return Fund, LP
Investment Holdings [Line Items]
Amount	$ 58,148	$ 54,147
Percentage of ownership	20.00%	20.00%
Growth Partners, LP
Investment Holdings [Line Items]
Amount	$ 59,886	$ 58,372
Percentage of ownership	33.00%	33.00%
CP I, LP
Investment Holdings [Line Items]
Amount	$ 1,001	$ 919
Percentage of ownership	10.70%	10.70%