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Summary of Significant Accounting Policies - Accounting standards (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |
Income tax benefit | $ (802) | $ (877) | $ (1,111)
Unearned premiums | 463 | 362 | 638
Equity securities | 1,160 | 509 |
Limited partnership interests | 6,740 | 5,814 |
Unrealized net capital gains and losses | 319 | 433 | (1,306)
Unrealized foreign currency translation adjustments | (47) | (10) | 58
Unrecognized pension and other postretirement benefit cost | 307 | (104) | 48
Accounting Standards Update 2017-09 |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |
Income tax benefit | 63 |  |
Accounting Standards Update 2014-09 |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |
Unearned premiums | 160 |  |
Deferred policy acquisition costs | 125 |  |
Accounting Standards Update 2016-01 |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |
Equity securities | 1,160 |  |
EMA limited partnerships |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |
Limited partnership interests | 5,410 | 4,530 |
EMA limited partnerships | Accounting Standards Update 2016-01 |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |
Limited partnership interests | 224 |  |
Accumulated other comprehensive income (loss) |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |
Reclassification of tax effects due to change in accounting principle | 49 | 0 | 0
Unrealized net capital gains and losses | 319 | 433 | (1,306)
Unrealized foreign currency translation adjustments | (47) | (10) | 58
Unrecognized pension and other postretirement benefit cost | 307 | (104) | 48
Retained income |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |
Reclassification of tax effects due to change in accounting principle | (49) | $ 0 | $ 0
Unrealized net capital gains and losses |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |
Reclassification of tax effects due to change in accounting principle | 290 |  |
Unrealized foreign currency translation adjustments |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |
Reclassification of tax effects due to change in accounting principle | (6) |  |
Unrecognized pension and other postretirement benefit cost |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |
Reclassification of tax effects due to change in accounting principle | $ (235) |  |

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- Definition The amount of unrealized gains (losses) included in accumulated other comprehensive income on equity securities which are categorized neither as held-to-maturity nor trading securities, as a result of fair value exceeding (under) its amortized cost, as of the balance sheet date. + References No definition available. + Details
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- Definition Amount of increase (decrease) in accumulated other comprehensive income (AOCI) for reclassification to retained earnings of tax effect from remeasurement of deferred tax pursuant to Tax Cuts and Jobs Act of 2017. + References No definition available. + Details
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- Definition Amount of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 9 -Subparagraph (a),(b) -URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32639-109319 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.4-08.(h)) -URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690 + Details
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- Definition Total investments in (A) an entity in which the entity has significant influence, but does not have control, (B) subsidiaries that are not required to be consolidated and are accounted for using the equity and or cost method, and (C) an entity in which the reporting entity shares control of the entity with another party or group. Includes long-term advances receivable from a party that is affiliated with the reporting entity by means of direct or indirect ownership. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.12) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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