Related party transaction (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Related Party Transaction [Line Items]
Ceded Premiums Written	$ 7,441	$ 7,849	$ 7,389
Losses and loss expenses	21,710	18,730	18,067
Insurance and reinsurance balances payable	6,708	6,184
Net premiums written	$ 33,820	$ 32,275	30,579
Bermuda \| ABR Reinsurance Capital Holdings Ltd. [Member]
Related Party Transaction [Line Items]
Ownership Percentage	15.60%	12.00%
Starr Technical Risk Agency and Affiliates [Member]
Related Party Transaction [Line Items]
Premiums Written, Gross	$ 507	$ 394	411
Ceded Premiums Written	253	207	188
Paid commissions	97	77	84
Commissions received	59	46	42
Losses and loss expenses	170	185	188
Reinsurance Recoverable on Losses and Loss Expenses	432	440
Insurance and reinsurance balances payable	80	56
Minimum Amount of Program Business to be Written to Earn Profit Sharing	20
ABR Reinsurance Capital Holdings Ltd. [Member]
Related Party Transaction [Line Items]
Ceded Premiums Written	350	321	329
Commissions received	100	92	$ 96
Reinsurance Recoverable on Losses and Loss Expenses	806	674
Insurance and reinsurance balances payable	$ 67	$ 62
Warrants & Rights Outstanding	0.50%

Minimum Amount of Program Business to be Written to Earn Profit Sharing

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- Definition

Reinsurance Recoverable on Losses and Loss Expenses

+ References

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Value of outstanding derivative securities that permit the holder the right to purchase a percentage of equity from the issuer.

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Amount of written premiums ceded to other entities.

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The percentage of ownership of common stock or equity participation in the investee accounted for under the equity method of accounting.

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The expense incurred by an insurance company to persons or entities for generating or placing insurance or investment contracts with the company, usually determined as a percentage of policy or contract premiums. Excludes advances or draws to be applied against commissions earned.

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Income from agency and brokerage operations (includes sales of annuities and supplemental contracts); service charges, commissions, and fees from the sale of insurance and related services; and management fees from separate accounts, deferred annuities, and universal life products.

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Amount, after effects of policies assumed or ceded, of expense related to the provision for policy benefits and costs incurred.

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Amount of direct and assumed premiums written.

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Amount, after premiums ceded to other entities and premiums assumed by the entity, of premiums written.

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The carrying amount as of the balance sheet date of the known and estimated amounts owed to insurers under reinsurance treaties or other arrangements.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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