Related party transaction (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Related Party Transaction [Line Items]
Ceded Premiums Written	$ 8,912	$ 7,441	$ 7,849
Losses and loss expenses	21,980	21,710	18,730
Insurance and reinsurance balances payable	$ 7,243	$ 6,708
Bermuda \| ABR Reinsurance Capital Holdings Ltd. [Member]
Related Party Transaction [Line Items]
Ownership Percentage	17.10%	16.00%
Starr Technical Risk Agency and Affiliates [Member]
Related Party Transaction [Line Items]
Minimum Amount of Program Business to be Written to Earn Profit Sharing	$ 20
Premiums Written, Gross	592	$ 507	394
Ceded Premiums Written	321	253	207
Paid commissions	114	97	77
Commissions received	73	59	46
Losses and loss expenses	157	170	185
Reinsurance Recoverable on Losses and Loss Expenses	516	432
Insurance and reinsurance balances payable	88	80
ABR Reinsurance Capital Holdings Ltd. [Member]
Related Party Transaction [Line Items]
Ceded Premiums Written	442	350	321
Commissions received	133	100	$ 92
Reinsurance Recoverable on Losses and Loss Expenses	963	806
Insurance and reinsurance balances payable	$ 107	$ 67
Warrants & Rights Outstanding	0.50%
BlackRock, Inc.
Related Party Transaction [Line Items]
Revenue from Related Parties	$ 11

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

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Reinsurance Recoverable on Losses and Loss Expenses

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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 of revenue, fees and commissions earned from transactions between (a) a parent company and its subsidiaries; (b) subsidiaries of a common parent; (c) an entity and trusts for the benefit of employees, for example, but not limited to, pension and profit-sharing trusts that are managed by or under the trusteeship of the entity's management; (d) an entity and its principal, owners, management, or members of their immediate families; and (e) affiliates.

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