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
	Mar. 31, 2018	Mar. 31, 2017
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	$ 122,250	$ 130,853
Total Realized and Unrealized Gains Included in Income	18,693	6,609
Sales	(12,128)	(17,086)
Issuances	4,899	4,030
Distributions and Other Settlements	0	0
Transfers Out of Level 3	(1,207)	(167)
Ending Balance	132,507	124,239
Equity warrant assets | Other assets
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	121,331	128,813
Total Realized and Unrealized Gains Included in Income	18,611	6,609
Sales	(12,128)	(17,086)
Issuances	4,899	4,030
Distributions and Other Settlements	0	0
Transfers Out of Level 3	(1,207)	(167)
Ending Balance	131,506	122,199
Non-marketable securities | Fair value accounting | Unconsolidated venture capital and private equity fund investments (1)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	919	2,040
Total Realized and Unrealized Gains Included in Income	82	0
Sales	0	0
Issuances	0	0
Distributions and Other Settlements	0	0
Transfers Out of Level 3	0	0
Ending Balance	$ 1,001	$ 2,040