Amounts of Gross Realized Capital Gains and Gross Realized Capital Losses (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Investments Debt And Equity Securities [Abstract]
Gross realized capital gains	$ 59,000	$ 12,100
Gross realized capital losses	(46,000)	(7,700)
Net realized capital gains	$ 13,026	$ 4,442