Reinsurance - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Effects of Reinsurance [Line Items]
Reinsurance collateral	$ 3,200	$ 3,400
Reinsurance recoveries	2,600	1,400	$ 1,500
Berkshire Hathaway Group [Member]
Effects of Reinsurance [Line Items]
Largest recoverables from a single reinsurer	2,400
Gateway Rivers Insurance Company [Member]
Effects of Reinsurance [Line Items]
Largest recoverables from a single reinsurer	284
Subsidiaries from Hartford Insurance Group [Member]
Effects of Reinsurance [Line Items]
Largest recoverables from a single reinsurer	207
Significant Captive Program [Member]
Effects of Reinsurance [Line Items]
Direct and ceded earned premiums	$ 3,300	2,600	2,200
Percentage reinsured of direct and ceded earned premiums	100.00%
Reinsurance recoveries	$ 2,300	$ 1,500	$ 712