Investment Securities - Components of Gains and Losses (Realized and Unrealized) on Investment Securities (Detail)	3 Months Ended	12 Months Ended
	Mar. 31, 2016 USD ($) Investment	Dec. 31, 2017 USD ($) Investment	Dec. 31, 2016 USD ($) Investment	Dec. 31, 2015 USD ($) Investment
Gain (Loss) on Investments [Line Items]
Gross gains on investment securities		$ 96,992,000	$ 87,500,000	$ 107,886,000
Gross losses on investment securities		(32,389,000)	(35,760,000)	(18,441,000)
Gains on investment securities, net		64,603,000	51,740,000	89,445,000
Other than Temporary Impairment Losses, Investments		$ 0	$ 0	$ 0
Cost method accounting | Venture capital and private equity fund investments
Gain (Loss) on Investments [Line Items]
Number of other-than-temporary impaired investments | Investment	26	24		22
Number of investments | Investment		235	252	267
Cost-method Investments, Other than Temporary Impairment	$ 600,000	$ 1,300,000		$ 600,000
Available-for-sale Securities
Gain (Loss) on Investments [Line Items]
Available-for-sale Securities, Gross Realized Gains		5,113,000	$ 15,051,000	2,971,000
Available-for-sale Securities, Gross Realized Losses		(10,302,000)	(2,856,000)	(1,770,000)
Non-marketable securities | Fair value accounting | Venture capital and private equity fund investments
Gain (Loss) on Investments [Line Items]
Gross gains on investment securities		34,093,000	25,041,000	32,399,000
Gross losses on investment securities		(6,907,000)	(19,077,000)	(9,210,000)
Non-marketable securities | Fair value accounting | Other venture capital investments
Gain (Loss) on Investments [Line Items]
Gross gains on investment securities		1,114,000	17,000	1,512,000
Gross losses on investment securities		(143,000)	(38,000)	(320,000)
Non-marketable securities | Fair value accounting | Other securities
Gain (Loss) on Investments [Line Items]
Gross gains on investment securities		991,000	691,000	9,180,000
Gross losses on investment securities		(750,000)	(781,000)	(1,559,000)
Non-marketable securities | Equity method accounting | Venture capital and private equity fund investments
Gain (Loss) on Investments [Line Items]
Gross gains on investment securities		15,013,000	10,834,000	26,415,000
Gross losses on investment securities		(541,000)	(6,764,000)	(909,000)
Non-marketable securities | Equity method accounting | Debt Securities
Gain (Loss) on Investments [Line Items]
Gross gains on investment securities		11,658,000	1,406,000	4,111,000
Gross losses on investment securities		(2,708,000)	(458,000)	(774,000)
Non-marketable securities | Equity method accounting | Other investments
Gain (Loss) on Investments [Line Items]
Gross gains on investment securities		3,181,000	15,739,000	2,791,000
Gross losses on investment securities		(9,457,000)	(4,857,000)	(3,146,000)
Non-marketable securities | Cost method accounting | Venture capital and private equity fund investments
Gain (Loss) on Investments [Line Items]
Gross gains on investment securities		21,718,000	18,428,000	25,908,000
Gross losses on investment securities		(1,312,000)	(591,000)	(729,000)
Non-marketable securities | Cost method accounting | Other investments
Gain (Loss) on Investments [Line Items]
Gross gains on investment securities		4,111,000	293,000	2,599,000
Gross losses on investment securities		$ (269,000)	$ (338,000)	$ (24,000)