Fair Value of Financial Instruments Additional Information about Level 3 Assets Measured at Fair Value on a Recurring Basis (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Equity warrant assets | Other Assets
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	$ 128,813	$ 135,168
Total Realized and Unrealized Gains (Losses) Included in Income	54,263	38,091
Sales	(74,769)	(56,643)
Issuances	14,537	13,405
Distributions and Other Settlements	0	0
Transfers Out of Level 3	(1,513)	(1,208)
Ending Balance		128,813	$ 135,168
Non-marketable securities | Fair value accounting | Other venture capital investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	2,040	2,040
Total Realized and Unrealized Gains (Losses) Included in Income	971	(21)
Sales	(2,092)	(4)
Issuances	0	0
Distributions and Other Settlements	0	25
Transfers Out of Level 3	0	0
Ending Balance		2,040	2,040
Fair Value, Measurements, Recurring
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	130,853	137,208	117,989
Total Realized and Unrealized Gains (Losses) Included in Income	55,234	38,070	72,594
Sales	(76,861)	(56,647)	(63,400)
Issuances	14,537	13,405	12,534
Distributions and Other Settlements	0	25	(87)
Transfers Out of Level 3	(1,513)	(1,208)	(2,422)
Ending Balance	122,250	130,853	137,208
Fair Value, Measurements, Recurring | Equity warrant assets | Other Assets
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	128,813	135,168	114,698
Total Realized and Unrealized Gains (Losses) Included in Income			71,402
Sales			(61,044)
Issuances			12,534
Distributions and Other Settlements			0
Transfers Out of Level 3			(2,422)
Ending Balance	121,331	128,813	135,168
Fair Value, Measurements, Recurring | Non-marketable securities | Fair value accounting | Other venture capital investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	2,040	2,040	3,291
Total Realized and Unrealized Gains (Losses) Included in Income			1,192
Sales			(2,356)
Issuances			0
Distributions and Other Settlements			(87)
Transfers Out of Level 3			0
Ending Balance	$ 919	$ 2,040	$ 2,040