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	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Investment Holdings [Line Items]
Amount	$ 126,467	$ 128,111
Strategic Investors Fund, LP
Investment Holdings [Line Items]
Amount	$ 12,733	$ 14,673
Percentage of ownership	12.60%	12.60%
Capital Preferred Return Fund, LP
Investment Holdings [Line Items]
Amount	$ 56,453	$ 54,147
Percentage of ownership	20.00%	20.00%
Growth Partners, LP
Investment Holdings [Line Items]
Amount	$ 56,280	$ 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%