Fair Value of Financial Instruments - Unrealized Gains Included in Earnings Attributable to Level 3 Assets Held (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Unrealized Gains (Losses) [Line Items]
Unrealized gains included in earnings attributable to Level 3 assets still held	$ 37,724	$ 10,730
Unrealized gains (losses) attributable to noncontrolling interests	143	(397)
Equity warrant assets
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Unrealized Gains (Losses) [Line Items]
Unrealized gains included in earnings attributable to Level 3 assets still held	37,564	11,174
Non-marketable securities | Unconsolidated venture capital and private equity fund investments | Fair value accounting
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Unrealized Gains (Losses) [Line Items]
Unrealized gains included in earnings attributable to Level 3 assets still held	$ 160	$ (444)