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Reinsurance - Property and casualty reinsurance recoverable (Details) - USD ($) $ in Millions | 12 Months Ended |  |
--- | --- | --- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014
Reinsurance Retention Policy [Line Items] |  |  |  |
Reinsurance recoverables, net | $ 8,921 | $ 8,745 |  |
Property-Liability |  |  |  |
Reinsurance Retention Policy [Line Items] |  |  |  |
Reinsurance recoverables, net | 6,570 | 6,280 |  |
Reinsurance recoverables related to losses paid by the Company and billed to reinsurers | 96 | 93 |  |
Reinsurance recoverables related to ceded unpaid losses | 6,471 | 6,184 | $ 5,892 | $ 5,694
Maximum amount due from any single reinsurer not separately disclosed | 31 | 35 |  |
Discontinued Lines and Coverages |  |  |  |
Reinsurance Retention Policy [Line Items] |  |  |  |
Allowance for uncollectible reinsurance | 70 | 84 |  |
Westport Insurance Corporation | Property-Liability |  |  |  |
Reinsurance Retention Policy [Line Items] |  |  |  |
Largest reinsurance recoverable from a single reinsurer | $ 61 | $ 61 |  |

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- Definition An allowance for the amount deemed uncollectible of an insurance receivable related to a policy by which an insurer indemnifies itself of some or all of the risk associated with another insurance policy. + References No definition available. + Details
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- Definition Represents the amount of reported and estimated unreported claims assumed by and receivable from the identified reinsurer with respect to disclosure of a concentration of credit risk. + References No definition available. + Details
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- Definition The maximum amount due or estimated to be due from a single reinsurer that is used as a threshold for disclosure. + References No definition available. + Details
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- Definition Estimated amount after valuation allowance of reinsurance recoverables, due from reinsurers for incurred claims not yet reported, and claims settlement expenses. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 310 -Section 45 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=6485522&loc=d3e21412-158489 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 310 -Section 45 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=6485522&loc=d3e21409-158489 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03(a)(6)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 + Details
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- Definition Amount, after valuation allowance, recoverable under reinsurance contracts including premium paid under reinsurance contracts. Examples include, but are not limited to, settled and unsettled claims, incurred but not reported losses, loss adjustment expense, premium paid, policy benefits and policy reserves. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 20 -Section 50 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=6480758&loc=d3e6824-158387 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 310 -Section 45 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=6485522&loc=d3e21409-158489 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.(a),6) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 + Details
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- Definition Amount, after valuation allowance, recoverable under reinsurance contracts for losses reported to and paid by the ceding insurer. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 310 -Section 45 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=6485522&loc=d3e21412-158489 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.(a),6) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 + Details
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- Definition 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. + References No definition available. + Details
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- Details
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