Investment Securities - Components of Gains and Losses (Realized and Unrealized) on Investment Securities (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Gain (Loss) on Investments [Line Items]
Net gains (losses) on non-marketable and other equity securities:	$ 0	$ 675
Gains and Losses on Non-Marketable and Other Securities	9,058	15,362
Non-marketable securities
Gain (Loss) on Investments [Line Items]
Gain (Loss) on Sale of Investments	22,490	(3,373)
Unrealized net gains on investment securities still held	31,548	11,989
Non-marketable securities | Fair value accounting
Gain (Loss) on Investments [Line Items]
Unrealized net gains on investment securities still held	(22,282)	(82)
Non-marketable securities | Fair value accounting | Consolidated venture capital and private equity fund investments
Gain (Loss) on Investments [Line Items]
Unrealized net gains on investment securities still held	11,647	6,463
Non-marketable securities | Fair value accounting | Unconsolidated venture capital and private equity fund investments (1)
Gain (Loss) on Investments [Line Items]
Unrealized net gains on investment securities still held	11,719	3,047
Non-marketable securities | Fair value accounting | Other securities
Gain (Loss) on Investments [Line Items]
Unrealized net gains on investment securities still held	1,741	3,373
Non-marketable securities | Equity method accounting | Consolidated venture capital and private equity fund investments
Gain (Loss) on Investments [Line Items]
Unrealized net gains on investment securities still held	9,569	3,734
Non-marketable securities | Equity method accounting | Other securities
Gain (Loss) on Investments [Line Items]
Unrealized net gains on investment securities still held	(1,037)	(742)
Non-marketable securities | Equity method accounting | Debt funds
Gain (Loss) on Investments [Line Items]
Unrealized net gains on investment securities still held	$ (2,299)	$ (431)