Investment Securities - Carrying Value and Ownership Percentage of Investments (Detail) - Non-marketable securities - Equity method accounting - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Consolidated venture capital and private equity fund investments
Investment Holdings [Line Items]
Non-marketable securities, equity method accounting	$ 112,537	$ 89,809
Consolidated venture capital and private equity fund investments | Strategic Investors Fund II, LP
Investment Holdings [Line Items]
Non-marketable securities, equity method accounting	$ 4,739	$ 6,342
Percentage of ownership	8.60%	8.60%
Consolidated venture capital and private equity fund investments | Strategic Investors Fund III, LP
Investment Holdings [Line Items]
Non-marketable securities, equity method accounting	$ 18,176	$ 18,758
Percentage of ownership	5.90%	5.90%
Consolidated venture capital and private equity fund investments | Strategic Investors Fund IV, LP
Investment Holdings [Line Items]
Non-marketable securities, equity method accounting	$ 29,445	$ 25,551
Percentage of ownership	5.00%	5.00%
Consolidated venture capital and private equity fund investments | Strategic Investors Fund V funds
Investment Holdings [Line Items]
Non-marketable securities, equity method accounting	$ 24,245	$ 16,856
Consolidated venture capital and private equity fund investments | CP II, LP
Investment Holdings [Line Items]
Non-marketable securities, equity method accounting	$ 6,865	$ 6,700
Percentage of ownership	5.10%	5.10%
Consolidated venture capital and private equity fund investments | CP II, LP | Indirect ownership interest
Investment Holdings [Line Items]
Percentage of ownership	3.80%
Consolidated venture capital and private equity fund investments | CP II, LP | Direct ownership interest
Investment Holdings [Line Items]
Percentage of ownership	1.30%
Consolidated venture capital and private equity fund investments | Other venture capital and private equity fund investments
Investment Holdings [Line Items]
Non-marketable securities, equity method accounting	$ 29,067	$ 15,602
Debt funds
Investment Holdings [Line Items]
Non-marketable securities, equity method accounting	$ 5,241	21,183
Debt funds | Indirect ownership interest
Investment Holdings [Line Items]
Percentage of ownership	4.00%
Debt funds | Direct ownership interest
Investment Holdings [Line Items]
Percentage of ownership	11.50%
Debt funds | Gold Hill Capital 2008, LP
Investment Holdings [Line Items]
Non-marketable securities, equity method accounting	$ 3,267	$ 18,690
Percentage of ownership	15.50%	15.50%
Debt funds | Other debt funds
Investment Holdings [Line Items]
Non-marketable securities, equity method accounting	$ 1,974	$ 2,493
Other investments
Investment Holdings [Line Items]
Non-marketable securities, equity method accounting	125,632	111,198
Other investments | SPD Silicon Valley Bank Co., Ltd.
Investment Holdings [Line Items]
Non-marketable securities, equity method accounting	$ 75,314	$ 75,337
Percentage of ownership	50.00%	50.00%
Other investments | Other investments
Investment Holdings [Line Items]
Non-marketable securities, equity method accounting	$ 50,318	$ 35,861