Amount of Gross Realized Capital Gains and Gross Realized Capital Losses of Available For Sale Securities (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Gain (Loss) on Investments [Line Items]
Gross realized capital gains	$ 344,700	$ 424,200	$ 301,100
Gross realized capital losses	(281,500)	(210,300)	(54,000)
Net realized capital gains	$ 63,205	$ 213,897	$ 247,058