Summary of Calculation of Realized Gains and Losses (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Investments Debt And Equity Securities [Abstract]
Equity securities, Total net realized investment gains (losses)	$ (1,465)	$ 4,843	$ 17,860	$ 2,534
Less: Net gains (losses) recognized during the period on equity securities sold during the period	(614)	2,096	9,836	3,958
Unrealized gains and (losses) recognized during the reporting period on equity securities still held at the reporting date	$ (851)	$ 2,747	$ 8,024	$ (1,424)