Reinsurance - Voluntary Reinsurance Receivables by Financial Strength (Details) - USD ($) $ in Millions	Dec. 31, 2024	Dec. 31, 2023
Voluntary Reinsurance Receivables [Abstract]
Voluntary reinsurance receivables	$ 5,491
Reinsurance collateral	3,400	$ 3,600
Subsidiaries of Berkshire Hathaway Insurance Group
Voluntary Reinsurance Receivables [Abstract]
Voluntary reinsurance receivables	1,800
Cavello Bay Reinsurance Limited
Voluntary Reinsurance Receivables [Abstract]
Voluntary reinsurance receivables	559
Gateway Rivers Insurance Company
Voluntary Reinsurance Receivables [Abstract]
Voluntary reinsurance receivables	371
A- to A++
Voluntary Reinsurance Receivables [Abstract]
Voluntary reinsurance receivables	4,585
B- to B++
Voluntary Reinsurance Receivables [Abstract]
Voluntary reinsurance receivables	892
Insolvent
Voluntary Reinsurance Receivables [Abstract]
Voluntary reinsurance receivables	$ 14