Amount of Gross Realized Capital Gains and Gross Realized Capital Losses of Available For Sale Securities (Detail) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Gain (Loss) on Investments [Line Items]
Gross realized capital gains	$ 61,600,000	$ 63,700,000	$ 215,400,000	$ 165,500,000
Gross realized capital losses	(2,200,000)	(45,900,000)	(17,700,000)	(69,900,000)
Net realized capital gains	$ 59,368,000	$ 17,762,000	$ 197,728,000	$ 95,641,000