Amounts of Gross Realized Capital Gains and Gross Realized Capital Losses (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Investments Debt And Equity Securities [Abstract]
Gross realized capital gains	$ 21,400	$ 12,600	$ 108,800	$ 38,300
Gross realized capital (losses)	(4,600)	(8,700)	(117,100)	(17,500)
Net realized capital gains	$ 16,751	$ 3,957	$ (8,323)	$ 20,753