Investment Securities - Components of Gains and Losses (Realized and Unrealized) on Investment Securities (Detail) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017 USD ($) Investment	Sep. 30, 2016 USD ($) Investment	Sep. 30, 2017 USD ($) Investment	Sep. 30, 2016 USD ($) Investment	Dec. 31, 2016 Investment
Gain (Loss) on Investments [Line Items]
Gross gains on investment securities:	$ 19,852	$ 26,328	$ 63,725	$ 67,752
Gross losses on investment securities	(4,614)	(3,150)	(14,887)	(25,988)
Gains on investment securities, net	15,238	23,178	48,838	41,764
Cost method accounting | Venture capital and private equity fund investments
Gain (Loss) on Investments [Line Items]
Recognized other-than-temporary impairment (OTTI) losses	$ 300	$ 100	$ 900	$ 500
Number of other-than-temporary impaired investments | Investment	8	5	21	21
Number of investments | Investment	237	255	237	255	252
Available-for-sale Securities
Gain (Loss) on Investments [Line Items]
Gross gains on investment securities:	$ 38	$ 84	$ 1,131	$ 14,238
Gross losses on investment securities	(139)	(99)	(747)	(2,671)
Non-marketable securities | Fair value accounting | Venture capital and private equity fund investments
Gain (Loss) on Investments [Line Items]
Gross gains on investment securities:	5,308	6,030	24,788	16,377
Gross losses on investment securities	(835)	(2,122)	(4,139)	(15,958)
Non-marketable securities | Fair value accounting | Other venture capital investments
Gain (Loss) on Investments [Line Items]
Gross gains on investment securities:	0	4	0	17
Gross losses on investment securities	0	0	(143)	(38)
Non-marketable securities | Fair value accounting | Other securities
Gain (Loss) on Investments [Line Items]
Gross gains on investment securities:	569	271	841	639
Gross losses on investment securities	(182)	(100)	(561)	(507)
Non-marketable securities | Equity method accounting | Venture capital and private equity fund investments
Gain (Loss) on Investments [Line Items]
Gross gains on investment securities:	4,542	5,679	11,245	9,351
Gross losses on investment securities	(223)	(444)	(535)	(4,465)
Non-marketable securities | Equity method accounting | Debt funds
Gain (Loss) on Investments [Line Items]
Gross gains on investment securities:	4,222	295	5,388	1,259
Gross losses on investment securities	(1,777)	(129)	(2,692)	(458)
Non-marketable securities | Equity method accounting | Other investments
Gain (Loss) on Investments [Line Items]
Gross gains on investment securities:	215	7,487	1,736	11,528
Gross losses on investment securities	(1,148)	(205)	(4,899)	(1,161)
Non-marketable securities | Cost method accounting | Venture capital and private equity fund investments
Gain (Loss) on Investments [Line Items]
Gross gains on investment securities:	4,956	6,328	14,985	14,180
Gross losses on investment securities	(259)	(51)	(914)	(492)
Non-marketable securities | Cost method accounting | Other investments
Gain (Loss) on Investments [Line Items]
Gross gains on investment securities:	2	150	3,611	163
Gross losses on investment securities	$ (51)	$ 0	$ (257)	$ (238)