Investments - Gross Realized Gains and Losses on Sale of Debt Securities (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Investments Debt And Equity Securities [Abstract]
Gross realized gains on debt securities	$ 24	$ 91	$ 49	$ 148
Gross realized losses on debt securities	(50)	(28)	(365)	(179)
Total gain (loss) recognized, net	$ (26)	$ 63	$ (316)	$ (31)