Fair Value of Financial Instruments - Unrealized Gains Included in Earnings Attributable to Level 3 Assets Held (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 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	$ 15,841	$ 17,827	$ 31,036	$ 23,591
Unrealized gains (losses) attributable to noncontrolling interests	0	0	73	(127)
Equity warrant assets | Other 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	15,841	17,827	30,954	23,734
Unconsolidated venture capital and private equity fund investments | Non-marketable securities | 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	$ 0	$ 0	$ 82	$ (143)