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Summary of Significant Accounting Policies - Accounting standards (Details) - USD ($) $ in Millions | 12 Months Ended |  |  |  |  |
--- | --- | --- | --- | --- | --- | ---
Dec. 31, 2018 | Jan. 01, 2018 | Dec. 31, 2017 | Dec. 31, 2016 | Jan. 01, 2016 | Dec. 31, 2015
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |
Equity securities |  |  | $ 1,160 |  |  |
Unrealized gain | $ 910 |  |  |  |  |
Limited partnership interests | 7,505 |  | 6,740 |  |  |
Unearned premiums | 14,510 |  | 13,473 |  |  |
Deferred policy acquisition costs | 4,784 |  | 4,191 | $ 3,954 |  | $ 3,861
Accounting Standards Update 2016-01 |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |
Equity securities | 1,160 |  | 1,160 |  |  |
EMA limited partnerships |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |
Limited partnership interests | 5,730 |  | 5,410 |  |  |
EMA limited partnerships | Accounting Standards Update 2016-01 |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |
Limited partnership interests |  |  | 224 |  |  |
Retained income |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |
Cumulative effect of change in accounting principle |  |  | 1,088 | 0 | $ 1,600 | 0
Accumulated other comprehensive income (loss) |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |
Cumulative effect of change in accounting principle |  |  | (910) | $ 0 |  | $ 0
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 |  |  |  |
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 |  |  |
Accounting Standards Update 2016-02 |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |
Lease, Right-of-Use Asset | 500 |  |  |  |  |
Lease, Liability | 500 |  |  |  |  |
Accounting Standards Update 2016-02 | Retained income |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |
Cumulative effect of change in accounting principle | $ 20 |  |  |  |  |

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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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