Amounts of Gross Realized Capital Gains and Gross Realized Capital Losses (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Schedule of Gross Gains (Losses) [Line Items]
Gross realized capital gains	$ 16,900	$ 47,000	$ 83,300	$ 189,700
Gross realized capital losses	(700)	(14,100)	(16,100)	(87,900)
Net realized capital gains	$ 16,230	$ 32,921	$ 67,197	$ 101,840