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Related party transaction (Details) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2017 | Sep. 30, 2017 | Jun. 30, 2017 | Mar. 31, 2017 | Dec. 31, 2016 | Sep. 30, 2016 | Jun. 30, 2016 | Mar. 31, 2016 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Related Party Transaction [Line Items] |  |  |  |  |  |  |  |  |  |  |
Other investments | $ 4,672 |  |  |  | $ 4,519 |  |  |  | $ 4,672 | $ 4,519 |
Net premiums written |  |  |  |  |  |  |  |  | 29,244 | 28,145 | $ 17,713
Losses and loss expenses | 4,272 | $ 6,247 | $ 4,146 | $ 3,789 | 3,855 | $ 4,269 | $ 4,254 | $ 3,674 | 18,454 | 16,052 | 9,484
Insurance and reinsurance balances payable | 5,868 |  |  |  | 5,637 |  |  |  | 5,868 | 5,637 |
Reinsurance recoverable on losses and loss expenses | 15,034 |  |  |  | 13,577 |  |  |  | 15,034 | 13,577 |
Ceded Premiums Written |  |  |  |  |  |  |  |  | 7,132 | 6,838 | 6,098
All Related Party [Member] |  |  |  |  |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |  |  |  |  |
Other investments | 86 |  |  |  |  |  |  |  | 86 |  |
Starr Technical Risk Agency and Affiliates [Member] |  |  |  |  |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |  |  |  |  |
Paid commissions |  |  |  |  |  |  |  |  | 101 | 145 | 60
Losses and loss expenses |  |  |  |  |  |  |  |  | 438 | 313 | 137
Insurance and reinsurance balances payable | 44 |  |  |  | 72 |  |  |  | 44 | 72 |
Reinsurance recoverable on losses and loss expenses | 557 |  |  |  | 412 |  |  |  | 557 | 412 |
Gross premiums written |  |  |  |  |  |  |  |  | 464 | 658 | 305
Ceded Premiums Written |  |  |  |  |  |  |  |  | 175 | 208 | 78
Minimum Amount of Program Business to be Written to Earn Profit Sharing |  |  |  |  |  |  |  |  | 20 |  |
Ceded Commissions |  |  |  |  |  |  |  |  | 37 | 56 | $ 19
ABR Reinsurance Capital Holdings Ltd. [Member] |  |  |  |  |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |  |  |  |  |
Insurance and reinsurance balances payable | 51 |  |  |  | 53 |  |  |  | 51 | 53 |
Reinsurance recoverable on losses and loss expenses | $ 365 |  |  |  | $ 148 |  |  |  | 365 | 148 |
Ceded Premiums Written |  |  |  |  |  |  |  |  | 342 | 288 |
Fees and Commissions, Other |  |  |  |  |  |  |  |  | $ 94 | $ 66 |
Equity Method Investment, Ownership Percentage |  |  |  |  | 11.30% |  |  |  |  | 11.30% |

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- Definition Amount of written premiums ceded to other entities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 605 -Section 50 -Paragraph 1 -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 605 -Section 50 -Paragraph 1 -Subparagraph (b)(3) -URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547 + Details
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