Investments - Transfers in and Out of Each of the Valuation Levels of Fair Values (Detail) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Quoted Prices in Active Markets for Identical Assets (Level 1)
Transfers In And Out Of Each Of The Valuation Levels Of Fair Values Of Investments[Line Items]
In	$ 14,572	$ 0
Out	0	0
Net	14,572	0
Significant Other Observable Inputs (Level 2)
Transfers In And Out Of Each Of The Valuation Levels Of Fair Values Of Investments[Line Items]
In	0	0
Out	(14,572)	0
Net	(14,572)	0
Significant Unobservable Inputs (Level 3)
Transfers In And Out Of Each Of The Valuation Levels Of Fair Values Of Investments[Line Items]
In	0	0
Out	0	0
Net	$ 0	$ 0