Amounts of Gross Realized Capital Gains and Gross Realized Capital Losses (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Schedule of Gross Gains (Losses) [Line Items]
Gross realized capital gains	$ 13,600	$ 6,500	$ 25,700	$ 66,400
Gross realized capital losses	(1,200)		(8,900)	(15,400)
Net realized capital gains	$ 12,354	$ 6,462	$ 16,796	$ 50,967