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		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	$ 130,849	$ 129,851	$ 130,853	$ 137,208
Total Realized and Unrealized Gains (Losses) Included in Income	24,354	21,096	41,406	33,094
Sales	(17,412)	(10,686)	(40,998)	(34,280)
Issuances	3,622	5,251	11,071	9,842
Distributions and Other Settlements	0	0	0	25
Transfers Out of Level 3	(441)	(252)	(1,360)	(629)
Ending Balance	140,972	145,260	140,972	145,260
Equity warrant assets | Other assets
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	128,952	127,811	128,813	135,168
Total Realized and Unrealized Gains (Losses) Included in Income	24,354	21,092	41,549	33,115
Sales	(17,412)	(10,682)	(40,998)	(34,276)
Issuances	3,622	5,251	11,071	9,842
Distributions and Other Settlements	0	0	0	0
Transfers Out of Level 3	(441)	(252)	(1,360)	(629)
Ending Balance	139,075	143,220	139,075	143,220
Non-marketable securities | Fair value accounting | Other venture capital investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	1,897	2,040	2,040	2,040
Total Realized and Unrealized Gains (Losses) Included in Income	0	4	(143)	(21)
Sales	0	(4)	0	(4)
Issuances	0	0	0	0
Distributions and Other Settlements	0	0	0	25
Transfers Out of Level 3	0	0	0	0
Ending Balance	$ 1,897	$ 2,040	$ 1,897	$ 2,040