Investment Securities - Components of Gains and Losses (Realized and Unrealized) on Investment Securities (Detail) $ in Thousands	3 Months Ended
	Mar. 31, 2017 USD ($) Investment	Mar. 31, 2016 USD ($) Investment	Dec. 31, 2016 Investment
Gain (Loss) on Investments [Line Items]
Gross gains on investment securities	$ 19,538	$ 10,389
Gross losses on investment securities	(3,568)	(15,073)
Gains (losses) on investment securities, net	15,970	(4,684)
Cost method accounting | Venture capital and private equity fund investments
Gain (Loss) on Investments [Line Items]
Recognized other-than-temporary impairment (OTTI) losses	$ 100	$ 200
Number of other-than-temporary impaired investments | Investment	5	10
Number of investments | Investment	249	267	252
Available-for-sale Securities
Gain (Loss) on Investments [Line Items]
Gross gains on investment securities	$ 675	$ 1,753
Gross losses on investment securities	(67)	(2,499)
Non-marketable securities | Fair value accounting | Venture capital and private equity fund investments
Gain (Loss) on Investments [Line Items]
Gross gains on investment securities	7,082	2,823
Gross losses on investment securities	(619)	(7,893)
Non-marketable securities | Fair value accounting | Other venture capital investments
Gain (Loss) on Investments [Line Items]
Gross gains on investment securities	0	8
Gross losses on investment securities	0	(38)
Non-marketable securities | Fair value accounting | Other securities
Gain (Loss) on Investments [Line Items]
Gross gains on investment securities	156	63
Gross losses on investment securities	(238)	(157)
Non-marketable securities | Equity method accounting | Venture capital and private equity fund investments
Gain (Loss) on Investments [Line Items]
Gross gains on investment securities	3,917	1,653
Gross losses on investment securities	(183)	(3,555)
Non-marketable securities | Equity method accounting | Debt funds
Gain (Loss) on Investments [Line Items]
Gross gains on investment securities	238	900
Gross losses on investment securities	(669)	(45)
Non-marketable securities | Equity method accounting | Other investments
Gain (Loss) on Investments [Line Items]
Gross gains on investment securities	742	851
Gross losses on investment securities	(1,484)	(644)
Non-marketable securities | Cost method accounting | Venture capital and private equity fund investments
Gain (Loss) on Investments [Line Items]
Gross gains on investment securities	3,159	2,338
Gross losses on investment securities	(112)	(171)
Non-marketable securities | Cost method accounting | Other investments
Gain (Loss) on Investments [Line Items]
Gross gains on investment securities	3,569	0
Gross losses on investment securities	$ (196)	$ (71)