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Business Segments (Revenues by Line of Business) (Details) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2017 | Sep. 30, 2017 | Jun. 30, 2017 | Mar. 31, 2017 | Dec. 31, 2016 | Sep. 30, 2016 | Jun. 30, 2016 | Mar. 31, 2016 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Revenues | $ 2,448 | $ 2,398 | $ 2,366 | $ 2,330 | $ 2,390 | $ 2,433 | $ 2,348 | $ 2,195 | $ 9,542 | $ 9,366 | $ 9,101
Total operating revenues |  |  |  |  |  |  |  |  | 9,449 | 9,316 | 9,168
Net realized investment gains (losses) |  |  |  |  |  |  |  |  | 93 | 50 | (67)
Loss on early redemption | 42 |  |  |  |  |  |  |  | 42 |  |
Management & Professional Liability |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  | 2,591 | 2,617 | 2,647
Surety |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  | 541 | 529 | 504
Warranty & Alternative Risks |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  | 551 | 511 | 461
Specialty |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  | 3,683 | 3,657 | 3,612
Middle Market |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  | 1,907 | 1,768 | 1,641
Small Business |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  | 480 | 582 | 621
Other Commercial Insurance |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  | 1,126 | 1,124 | 1,156
Commercial |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  | 3,513 | 3,474 | 3,418
Canada |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  | 224 | 203 | 213
CNA Europe |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  | 326 | 314 | 311
Hardy |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  | 359 | 340 | 331
International |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  | 909 | 857 | 855
Life & Group |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  | 1,323 | 1,301 | 1,259
Corporate & Other |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  | 22 | 28 | 29
Eliminations |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  | (1) | $ (1) | $ (5)
Senior Notes, Due November 2019 |  |  |  |  |  |  |  |  |  |  |
Face amount | $ 350 |  |  |  |  |  |  |  | $ 350 |  |

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