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General (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended |  |  |  |
--- | --- | --- | --- | --- | --- | ---
Sep. 30, 2018 | Sep. 30, 2018 | Dec. 31, 2018 | Jan. 01, 2018 | Dec. 31, 2017 | Dec. 31, 2016
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |
Equity securities, unrealized gain |  | $ 910 |  |  |  |
Unearned premiums | $ 14,408 | 14,408 |  |  | $ 13,473 |
Deferred policy acquisition costs | 4,667 | 4,667 |  |  | 4,191 |
Tax Legislation expense | 31 |  |  |  |  |
Assets | 114,490 | 114,490 |  |  | 112,422 |
Liabilities | $ 90,857 | $ 90,857 |  |  | 89,871 |
Difference between Revenue Guidance in Effect before and after Topic 606 | Accounting Standards Update 2014-09 |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |
Unearned premiums |  |  |  | $ 160 |  |
Deferred policy acquisition costs |  |  |  | $ 160 |  |
Retained income |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |
Cumulative effect of change in accounting principle |  |  |  |  | 1,088 | $ 0
Retained income | Accounting Standards Update 2016-01 |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |
Cumulative effect of change in accounting principle |  |  |  |  | 1,160 |
Limited partnership interests | Accounting Standards Update 2016-01 |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |
Cumulative effect of change in accounting principle |  |  |  |  | 224 |
Limited partnership interests | Retained income | Accounting Standards Update 2016-01 |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |
Cumulative effect of change in accounting principle |  |  |  |  | $ 177 |
Forecast | Accounting Standards Update 2017-13 |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |
Assets |  |  | $ 525 |  |  |
Liabilities |  |  | 525 |  |  |
Forecast | Retained income | Accounting Standards Update 2017-13 |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |
Cumulative effect of change in accounting principle |  |  | $ 20 |  |  |

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