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Insurance - Analysis of changes in the liability for losses and loss adjustment expenses, net of reinsurance (Details) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended |
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Dec. 31, 2018 | Sep. 30, 2018 | Jun. 30, 2018 | Mar. 31, 2018 | Dec. 31, 2017 | Sep. 30, 2017 | Jun. 30, 2017 | Mar. 31, 2017 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Analysis of changes in the liability for losses and loss adjustment expenses, net of reinsurance |  |  |  |  |  |  |  |  |  |  |  |  |  |
Balance at beginning of period |  |  |  | $ 9,678 |  |  |  | $ 8,563 | $ 9,678 |  | $ 8,563 | $ 8,127 |  |
Less reinsurance recoverables, net of allowance | $ 2,942 |  |  |  | $ 2,957 |  |  |  | 2,942 |  | 2,957 | 2,302 |  | $ 2,201
Net liability at beginning of period |  |  |  | 6,721 |  |  |  | 6,261 | 6,721 |  | 6,261 | 5,926 |  |
Liability for losses and loss adjustment expenses, period increase (decrease) [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Provision for losses and LAE occurring in the current year |  |  |  |  |  |  |  |  | 3,195 |  | 3,019 | 2,730 |  |
Net increase (decrease) in the provision for claims of prior years: |  |  |  |  |  |  |  |  |  |  |  |  |  |
Provision for claims of prior years related to special A&E charges, Neon exited lines development and other | (61) | $ (31) | $ (44) | $ (56) | (66) | $ 52 | $ (22) | $ (28) | (192) |  | (64) |  |  |
Total losses and LAE incurred |  |  |  |  |  |  |  |  | 3,003 |  | 2,955 | 2,762 |  |
Payments for losses and LAE of: |  |  |  |  |  |  |  |  |  |  |  |  |  |
Current year |  |  |  |  |  |  |  |  | (963) |  | (942) | (841) |  |
Prior years |  |  |  |  |  |  |  |  | (1,639) |  | (1,586) | (1,512) |  |
Total payments |  |  |  |  |  |  |  |  | (2,602) |  | (2,528) | (2,353) |  |
Reserves of businesses disposed |  |  |  |  |  |  |  |  | (319) | [1] | 0 | (40) | [1] |
Foreign currency translation and other |  |  |  |  |  |  |  |  | (4) |  | 33 | (34) |  |
Net liability at end of period | 6,799 |  |  |  | 6,721 |  |  |  | 6,799 |  | 6,721 | 6,261 |  |
Add back reinsurance recoverables, net of allowance | 2,942 |  |  |  | 2,957 |  |  |  | 2,942 |  | 2,957 | 2,302 |  |
Gross unpaid losses and LAE included in the balance sheet | $ 9,741 |  |  |  | $ 9,678 |  |  |  | 9,741 |  | 9,678 | 8,563 |  |
Special A&E charges |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net increase (decrease) in the provision for claims of prior years: |  |  |  |  |  |  |  |  |  |  |  |  |  |
Provision for claims of prior years related to special A&E charges, Neon exited lines development and other |  |  |  |  |  |  |  |  | 18 |  | 89 | 36 |  |
Neon exited lines charge |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net increase (decrease) in the provision for claims of prior years: |  |  |  |  |  |  |  |  |  |  |  |  |  |
Provision for claims of prior years related to special A&E charges, Neon exited lines development and other |  |  |  |  |  |  |  |  | 0 |  | (18) | 57 |  |
Other |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net increase (decrease) in the provision for claims of prior years: |  |  |  |  |  |  |  |  |  |  |  |  |  |
Provision for claims of prior years related to special A&E charges, Neon exited lines development and other |  |  |  |  |  |  |  |  | $ (210) |  | $ (135) | $ (61) |  |
[1] | Reflects the reinsurance to close transactions at Neon (discussed below).

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- Definition Amount of increase (decrease) in the cost of settling unpaid claims after deduction of reinsurance recoveries related to the disposal of a business. Includes, but is not limited to, claims which have been incurred but not reported. + References No definition available. + Details
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- Definition The amount needed to reflect the estimated ultimate cost of settling claims relating to casualty insurance insured events that have occurred on or before a particular date (ordinarily, the balance sheet date) and the amount needed to provide for the estimated ultimate cost required to investigate and settle claims relating to insured events that have occurred on or before a particular date (ordinarily, the balance sheet date), whether or not reported to the insurer at that date. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (cc) -URI http://asc.fasb.org/extlink&oid=99382578&loc=d3e14764-158437 Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=99382578&loc=d3e14764-158437 + Details
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- Definition Amount of foreign currency gain (loss) which decreases (increases) the cost of settling unpaid claims after deduction of reinsurance recoveries. Includes, but is not limited to, claims which have been incurred but not reported. + References No definition available. + Details
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- Definition Liability as of the balance sheet date for amounts representing estimated cost of settling unpaid claims under the terms of the underlying insurance policies, less estimated reinsurance recoveries on such claims. This includes an estimate for claims which have been incurred but not reported. Claim adjustment expenses represent the costs estimated to be incurred in the settlement of unpaid claims. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03(13)(a)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (cc) -URI http://asc.fasb.org/extlink&oid=99382578&loc=d3e14764-158437 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=99382578&loc=d3e14764-158437 + Details
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- Definition Amount, after valuation allowance, recoverable under reinsurance contracts for losses reported to the ceding insurer but not yet paid and amounts expected for incurred losses and settlement expenses, which have not yet been reported to the ceding insurer. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 310 -Section 45 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=99404803&loc=d3e21412-158489 Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 20 -Section 50 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=99397103&loc=d3e6824-158387 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=99382578&loc=d3e14764-158437 Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (cc) -URI http://asc.fasb.org/extlink&oid=99382578&loc=d3e14764-158437 + Details
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