Investments - Transfers in and Out of Each of the Valuation Levels of Fair Values (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
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	$ 42,372	$ 45,344	$ 17,252
Out	(597)	0	(49,744)
Net	41,775	45,344	(32,492)
Significant Other Observable Inputs (Level 2)
Transfers In And Out Of Each Of The Valuation Levels Of Fair Values Of Investments[Line Items]
In	597	0	49,744
Out	(42,372)	(45,344)	(17,252)
Net	(41,775)	(45,344)	32,492
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	0
Out	0	0	0
Net	$ 0	$ 0	$ 0