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Reserve for Property-Liability Insurance Claims and Claims Expense - Incurred and claims expense narrative (Details) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Auto |  |  |
Increase (decrease) in claims and claims expense |  |  |
Net increase (decrease) to reserves | $ (490) | $ (155) | $ 30
Catastrophes |  |  |
Increase (decrease) in claims and claims expense |  |  |
Losses from catastrophes, net of recoveries | 3,230 | 2,570 | 1,720
Non-catastrophes loss reestimates | Homeowners |  |  |
Increase (decrease) in claims and claims expense |  |  |
Net increase (decrease) to reserves | (131) | (24) |
Catastrophe loss reestimates |  |  |
Increase (decrease) in claims and claims expense |  |  |
Net increase (decrease) to reserves | 18 | 6 | (15)
Catastrophe loss reestimates | Homeowners |  |  |
Increase (decrease) in claims and claims expense |  |  |
Net increase (decrease) to reserves |  |  | (24)
Reserve reestimates | Other personal lines |  |  |
Increase (decrease) in claims and claims expense |  |  |
Net increase (decrease) to reserves | 22 | 57 | 22
Reserve reestimates | Discontinued Lines and Coverages |  |  |
Increase (decrease) in claims and claims expense |  |  |
Net increase (decrease) to reserves |  | $ 105 | $ 53
Discontinued Lines and Coverages | Catastrophe loss reestimates |  |  |
Increase (decrease) in claims and claims expense |  |  |
Net increase (decrease) to reserves | $ 96 |  |

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- Definition Amount, after effects of reinsurance, of expense (reversal of expense) for claims incurred and costs incurred in the claim settlement process. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=65670824&loc=d3e14764-158437 + Details
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- Definition Amount, after effects of reinsurance, of expense (reversal of expense) for claims incurred in prior reporting periods and related claims settlement costs. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=65670824&loc=d3e14764-158437 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 235 -Section S99 -Paragraph 3 -Subparagraph (SX 210.12-18) -URI http://asc.fasb.org/extlink&oid=34713774&loc=d3e575016-122915 + Details
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