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, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	$ 124,239	$ 131,022	$ 130,853	$ 137,208
Total Realized and Unrealized Gains (Losses) Included in Income	10,443	4,848	17,052	11,997
Sales	(6,500)	(8,178)	(23,586)	(23,594)
Issuances	3,419	2,218	7,449	4,592
Distributions and Other Settlements	0	(5)	0	25
Transfers Out of Level 3	(752)	(54)	(919)	(377)
Ending Balance	130,849	129,851	130,849	129,851
Equity warrant assets | Other assets
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	122,199	128,982	128,813	135,168
Total Realized and Unrealized Gains (Losses) Included in Income	10,586	4,843	17,195	12,022
Sales	(6,500)	(8,178)	(23,586)	(23,594)
Issuances	3,419	2,218	7,449	4,592
Distributions and Other Settlements	0	0	0	0
Transfers Out of Level 3	(752)	(54)	(919)	(377)
Ending Balance	128,952	127,811	128,952	127,811
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	2,040	2,040	2,040	2,040
Total Realized and Unrealized Gains (Losses) Included in Income	(143)	5	(143)	(25)
Sales	0	0	0	0
Issuances	0	0	0	0
Distributions and Other Settlements	0	(5)	0	25
Transfers Out of Level 3	0	0	0	0
Ending Balance	$ 1,897	$ 2,040	$ 1,897	$ 2,040