Amount of Gross Realized Capital Gains and Gross Realized Capital Losses of Available For Sale Securities (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Gain (Loss) on Investments [Line Items]
Gross realized capital gains	$ 42,400	$ 61,600	$ 258,600	$ 215,400
Gross realized capital losses	(13,200)	(2,200)	(100,100)	(17,700)
Net realized capital gains	$ 29,202	$ 59,368	$ 158,513	$ 197,728