Reinsurance - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Effects of Reinsurance [Line Items]
Reinsurance collateral	$ 3,200	$ 2,900
Reinsurance recoveries	2,800	3,100	$ 3,000
Berkshire Hathaway Group [Member]
Effects of Reinsurance [Line Items]
Largest recoverables from a single reinsurer	2,200
Gateway Rivers Insurance Company [Member]
Effects of Reinsurance [Line Items]
Largest recoverables from a single reinsurer	278
Subsidiaries from Hartford Insurance Group [Member]
Effects of Reinsurance [Line Items]
Largest recoverables from a single reinsurer	233
Significant Captive Program [Member]
Effects of Reinsurance [Line Items]
Direct and ceded earned premiums	$ 3,700	3,900	3,900
Percentage reinsured of direct and ceded earned premiums	100.00%
Reinsurance recoveries	$ 1,900	$ 2,500	$ 2,600