Fair Value of Financial Instruments - Additional Information about Level 3 Assets Measured at Fair Value on a Recurring Basis (Details) - Measured on a recurring basis - Fair value accounting - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	$ 122,250	$ 130,853	$ 137,208
Total Realized and Unrealized Gains (Losses) Included in Income	88,439	55,234	38,070
Sales	(78,752)	(76,861)	(56,647)
Issuances	17,941	14,537	13,405
Distributions and Other Settlements	(297)	0	25
Transfers Out of Level 3	(3,303)	(1,513)	(1,208)
Ending Balance	146,278	122,250	130,853
Equity warrant assets
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	121,331	128,813	135,168
Total Realized and Unrealized Gains (Losses) Included in Income	87,982	54,263	38,091
Sales	(78,752)	(74,769)	(56,643)
Issuances	17,941	14,537	13,405
Distributions and Other Settlements	0	0	0
Transfers Out of Level 3	(3,303)	(1,513)	(1,208)
Ending Balance	145,199	121,331	128,813
Non-marketable securities | Unconsolidated venture capital and private equity fund investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	919	2,040	2,040
Total Realized and Unrealized Gains (Losses) Included in Income	457	971	(21)
Sales	0	(2,092)	(4)
Issuances	0	0	0
Distributions and Other Settlements	(297)	0	25
Transfers Out of Level 3	0	0	0
Ending Balance	$ 1,079	$ 919	$ 2,040