Amounts of Gross Realized Capital Gains and Gross Realized Capital Losses (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Investments Debt And Equity Securities [Abstract]
Gross realized capital gains	$ 28,500	$ 13,600	$ 87,400	$ 25,700
Gross realized capital (losses)	(66,600)	(1,200)	(112,500)	(8,900)
Net realized capital gains	$ (38,100)	$ 12,354	$ (25,074)	$ 16,796