Reinsurance - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Effects of Reinsurance [Line Items]
Reinsurance collateral	$ 3,400,000,000	$ 3,900,000,000
Reinsurance recoveries	1,400,000,000	1,500,000,000	1,400,000,000
Berkshire Hathaway Group [Member]
Effects of Reinsurance [Line Items]
Largest recoverables from a single reinsurer	2,600,000,000
Subsidiaries from Hartford Insurance Group [Member]
Effects of Reinsurance [Line Items]
Largest recoverables from a single reinsurer	244,000,000
Subsidiaries from Wilton Re [Member]
Effects of Reinsurance [Line Items]
Largest recoverables from a single reinsurer	185,000,000
Significant Captive Program [Member]
Effects of Reinsurance [Line Items]
Direct and ceded earned premiums	2,600,000,000	2,200,000,000	1,800,000,000
Percentage reinsured of direct and ceded earned premiums	100.00%
Reinsurance recoveries	$ 1,500,000,000	$ 712,000,000	$ 814,000,000