Investments - Summary of Proceeds Received and The Gross Realized Gains and Losses From Sales of Trading Equity Securities (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Debt and Equity Securities, FV-NI [Line Items]
Proceeds from sales of equity securities			$ 55,879	$ 26,309
Equity Securities [Member]
Debt and Equity Securities, FV-NI [Line Items]
Proceeds from sales of equity securities	$ 15,443	$ 5,407	55,879	26,309
Gross Realized Gains sales of equity securities	1,926	235	6,897	3,070
Gross Realized Losses sales of equity securities	$ (439)	$ (535)	$ (1,183)	$ (875)