Investment Securities - Net Gains on Non-marketable and Other Equity Securities (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Gain (Loss) on Securities [Line Items]
Total net gains on non-marketable and other equity securities	$ 32,193	$ 15,339	$ 77,365	$ 48,454
Non-marketable securities
Gain (Loss) on Securities [Line Items]
Unrealized net gains on non-marketable and other equity securities still held	31,836	15,388	98,171	45,099
Less: Net gains (losses) on non-marketable and other equity securities sold	357	(49)	(20,806)	3,355
Non-marketable securities | Fair value accounting | Consolidated venture capital and private equity fund investments
Gain (Loss) on Securities [Line Items]
Unrealized net gains on non-marketable and other equity securities still held	2,928	4,473	18,971	20,649
Non-marketable securities | Fair value accounting | Unconsolidated venture capital and private equity fund investments
Gain (Loss) on Securities [Line Items]
Unrealized net gains on non-marketable and other equity securities still held	6,240	4,697	37,095	13,928
Non-marketable securities | Fair value accounting | Other investments
Gain (Loss) on Securities [Line Items]
Unrealized net gains on non-marketable and other equity securities still held	2,509	(49)	4,310	3,354
Non-marketable securities | Equity method accounting | Consolidated venture capital and private equity fund investments
Gain (Loss) on Securities [Line Items]
Unrealized net gains on non-marketable and other equity securities still held	11,341	4,319	30,122	10,710
Non-marketable securities | Equity method accounting | Other investments
Gain (Loss) on Securities [Line Items]
Unrealized net gains on non-marketable and other equity securities still held	3,295	(933)	4,753	(3,163)
Non-marketable securities | Equity method accounting | Debt funds
Gain (Loss) on Securities [Line Items]
Unrealized net gains on non-marketable and other equity securities still held	1,473	2,445	(100)	2,696
Other equity securities | Fair value accounting
Gain (Loss) on Securities [Line Items]
Unrealized net gains on non-marketable and other equity securities still held	$ 4,407	$ 387	$ (17,786)	$ 280