Fair Value of Financial Instruments - Additional Information about Level 3 Assets Measured at Fair Value on a Recurring Basis (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	$ 132,507	$ 124,239	$ 122,250	$ 130,853
Total Realized and Unrealized Gains (Losses) Included in Income	18,249	10,443	36,942	17,052
Sales	(15,235)	(6,500)	(27,363)	(23,586)
Issuances	4,299	3,419	9,198	7,449
Distributions and Other Settlements	0	0	0	0
Transfers Out of Level 3	(1,066)	(752)	(2,273)	(919)
Ending Balance	138,754	130,849	138,754	130,849
Equity warrant assets | Other assets
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	131,506	122,199	121,331	128,813
Total Realized and Unrealized Gains (Losses) Included in Income	18,249	10,586	36,860	17,195
Sales	(15,235)	(6,500)	(27,363)	(23,586)
Issuances	4,299	3,419	9,198	7,449
Distributions and Other Settlements	0	0	0	0
Transfers Out of Level 3	(1,066)	(752)	(2,273)	(919)
Ending Balance	137,753	128,952	137,753	128,952
Non-marketable securities | Fair value accounting | Unconsolidated venture capital and private equity fund investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	1,001	2,040	919	2,040
Total Realized and Unrealized Gains (Losses) Included in Income	0	(143)	82	(143)
Sales	0	0	0	0
Issuances	0	0	0	0
Distributions and Other Settlements	0	0	0	0
Transfers Out of Level 3	0	0	0	0
Ending Balance	$ 1,001	$ 1,897	$ 1,001	$ 1,897