Amounts of Gross Realized Capital Gains and Gross Realized Capital Losses (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule of Gross Gains (Losses) [Line Items]
Gross realized capital gains	$ 236,700	$ 344,700	$ 424,200
Gross realized capital losses	(129,500)	(281,500)	(210,300)
Net realized capital gains	$ 107,222	$ 63,205	$ 213,897