Reinsurance - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Effects of Reinsurance [Line Items]
Reinsurance collateral	$ 3,000	$ 3,200
Reinsurance recoveries	3,000	2,600	$ 1,400
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	350
Subsidiaries from Hartford Insurance Group [Member]
Effects of Reinsurance [Line Items]
Largest recoverables from a single reinsurer	212
Significant Captive Program [Member]
Effects of Reinsurance [Line Items]
Direct and ceded earned premiums	$ 3,900	3,300	2,600
Percentage reinsured of direct and ceded earned premiums	100.00%
Reinsurance recoveries	$ 2,600	$ 2,300	$ 1,500