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Segments of Operations - Revenues by segment and sub-segment (Details) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
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
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Total premiums earned |  |  |  |  |  |  |  |  |  | $ 4,865 | $ 4,579 | $ 4,328
Net investment income |  |  |  |  |  |  |  |  |  | 2,094 | 1,831 | 1,696
Other income |  |  |  |  |  |  |  |  |  | 199 | 206 | 224
Revenues before realized gains (losses) |  |  |  |  |  |  |  |  |  | 7,416 | 6,860 | 6,477
Realized gains (losses) on securities |  |  |  |  |  |  |  |  |  | (266) | 5 | 19
Realized gains on subsidiaries |  |  |  |  |  |  |  |  |  | 0 | 0 | 2
Total revenues |  | $ 1,690 | $ 2,008 | $ 1,833 | $ 1,619 | $ 1,808 | $ 1,835 | $ 1,646 | $ 1,576 | 7,150 | 6,865 | 6,498
Property and casualty insurance |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Total premiums earned |  |  |  |  |  |  |  |  |  | 4,865 | 4,579 | 4,328
Net investment income |  |  |  |  |  |  |  |  |  | 438 | 362 | 350
Other income | [1] |  |  |  |  |  |  |  |  | 10 | 28 | 51
Revenues before realized gains (losses) |  |  |  |  |  |  |  |  |  | 5,313 | 4,969 | 4,729
Annuity |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Net investment income |  |  |  |  |  |  |  |  |  | 1,638 | 1,458 | 1,356
Other income |  |  |  |  |  |  |  |  |  | 107 | 103 | 103
Revenues before realized gains (losses) |  |  |  |  |  |  |  |  |  | 1,745 | 1,561 | 1,459
Other |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Revenues before realized gains (losses) |  |  |  |  |  |  |  |  |  | 358 | 330 | 289
Property and transportation | Property and casualty insurance |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Total premiums earned |  |  |  |  |  |  |  |  |  | 1,729 | 1,711 | 1,662
Specialty casualty | Property and casualty insurance |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Total premiums earned |  |  |  |  |  |  |  |  |  | 2,403 | 2,186 | 2,006
Specialty financial | Property and casualty insurance |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Total premiums earned |  |  |  |  |  |  |  |  |  | 598 | 576 | 557
Other specialty | Property and casualty insurance |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Total premiums earned |  |  |  |  |  |  |  |  |  | $ 135 | $ 106 | $ 103
[1] | Includes income of $32 million (before noncontrolling interest) from the sale of an apartment property in 2016.

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