Amounts of Gross Realized Capital Gains and Gross Realized Capital 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]
Gross realized capital gains	$ 12,600	$ 16,900	$ 38,300	$ 83,300
Gross realized capital losses	(8,700)	(700)	(17,500)	(16,100)
Net realized capital gains	$ 3,957	$ 16,230	$ 20,753	$ 67,197