Amounts of Gross Realized Capital Gains and Gross Realized Capital Losses (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Schedule of Gross Gains (Losses) [Line Items]
Gross realized capital gains	$ 60,300	$ 108,700
Gross realized capital losses	(15,800)	(49,100)
Net realized capital gains	$ 44,505	$ 59,651