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Segments of Operations - Revenues by segment and sub-segment (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2018 | Sep. 30, 2017 | Sep. 30, 2018 | Sep. 30, 2017
Segment Reporting Information [Line Items] |  |  |  |  |
Total premiums earned |  | $ 1,327 | $ 1,267 | $ 3,595 | $ 3,354
Net investment income |  | 527 | 471 | 1,552 | 1,366
Other income |  | 54 | 48 | 146 | 154
Revenues before realized gains (losses) |  | 1,974 | 1,847 | 5,488 | 5,058
Realized gains (losses) on securities |  | 34 | (12) | (28) | (1)
Total revenues |  | 2,008 | 1,835 | 5,460 | 5,057
Property and Casualty Insurance |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |
Total premiums earned |  | 1,327 | 1,267 | 3,595 | 3,354
Net investment income |  | 108 | 94 | 323 | 276
Other income | [1] | 4 | 1 | 8 | 21
Revenues before realized gains (losses) |  | 1,439 | 1,362 | 3,926 | 3,651
Annuity |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |
Net investment income |  | 413 | 375 | 1,219 | 1,082
Other income |  | 27 | 26 | 80 | 79
Revenues before realized gains (losses) |  | 440 | 401 | 1,299 | 1,161
Other |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |
Revenues before realized gains (losses) |  | 95 | 84 | 263 | 246
Specialty Property and transportation | Property and Casualty Insurance |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |
Total premiums earned |  | 526 | 527 | 1,250 | 1,226
Specialty casualty | Property and Casualty Insurance |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |
Total premiums earned |  | 616 | 568 | 1,790 | 1,613
Specialty financial | Property and Casualty Insurance |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |
Total premiums earned |  | 149 | 142 | 457 | 435
Other specialty | Property and Casualty Insurance |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |
Total premiums earned |  | $ 36 | $ 30 | $ 98 | $ 80
[1] | Includes income of $13 million (before noncontrolling interest) from the sale of a hotel in the first quarter of 2017.

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