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Investments - Gross unrealized losses on securities by investment category and length of time that have been in a continuous unrealized loss position (Details) - USD ($) $ in Millions | Dec. 31, 2017 | Dec. 31, 2016
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Fixed maturities |  |
Available-for-sale securities in a continuous unrealized loss position |  |
Unrealized Loss - Less than twelve months | $ (37) | $ (196)
Fair Value - Less than twelve months | $ 4,295 | $ 8,651
Fair Value as % of Cost - Less than twelve months | 99.00% | 98.00%
Unrealized Loss - Twelve months or more | $ (45) | $ (49)
Fair Value - Twelve months or more | $ 1,574 | $ 1,160
Fair Value as % of Cost - Twelve months or more | 97.00% | 96.00%
U.S. Government and government agencies |  |
Available-for-sale securities in a continuous unrealized loss position |  |
Unrealized Loss - Less than twelve months | $ 0 | $ (1)
Fair Value - Less than twelve months | $ 55 | $ 153
Fair Value as % of Cost - Less than twelve months | 100.00% | 99.00%
Unrealized Loss - Twelve months or more | $ (3) | $ (2)
Fair Value - Twelve months or more | $ 123 | $ 8
Fair Value as % of Cost - Twelve months or more | 98.00% | 80.00%
States, municipalities and political subdivisions |  |
Available-for-sale securities in a continuous unrealized loss position |  |
Unrealized Loss - Less than twelve months | $ (8) | $ (64)
Fair Value - Less than twelve months | $ 825 | $ 2,289
Fair Value as % of Cost - Less than twelve months | 99.00% | 97.00%
Unrealized Loss - Twelve months or more | $ (10) | $ (5)
Fair Value - Twelve months or more | $ 431 | $ 44
Fair Value as % of Cost - Twelve months or more | 98.00% | 90.00%
Foreign government |  |
Available-for-sale securities in a continuous unrealized loss position |  |
Unrealized Loss - Less than twelve months | $ 0 |
Fair Value - Less than twelve months | $ 4 |
Fair Value as % of Cost - Less than twelve months | 100.00% |
Unrealized Loss - Twelve months or more | $ 0 |
Fair Value - Twelve months or more | $ 0 |
Fair Value as % of Cost - Twelve months or more | 0.00% |
Residential MBS |  |
Available-for-sale securities in a continuous unrealized loss position |  |
Unrealized Loss - Less than twelve months | $ (1) | $ (7)
Fair Value - Less than twelve months | $ 118 | $ 502
Fair Value as % of Cost - Less than twelve months | 99.00% | 99.00%
Unrealized Loss - Twelve months or more | $ (5) | $ (6)
Fair Value - Twelve months or more | $ 118 | $ 162
Fair Value as % of Cost - Twelve months or more | 96.00% | 96.00%
Commercial MBS |  |
Available-for-sale securities in a continuous unrealized loss position |  |
Unrealized Loss - Less than twelve months | $ (1) | $ (2)
Fair Value - Less than twelve months | $ 67 | $ 121
Fair Value as % of Cost - Less than twelve months | 99.00% | 98.00%
Unrealized Loss - Twelve months or more | $ 0 | $ 0
Fair Value - Twelve months or more | $ 0 | $ 0
Fair Value as % of Cost - Twelve months or more | 0.00% | 0.00%
Asset-backed securities |  |
Available-for-sale securities in a continuous unrealized loss position |  |
Unrealized Loss - Less than twelve months | $ (7) | $ (29)
Fair Value - Less than twelve months | $ 1,195 | $ 1,737
Fair Value as % of Cost - Less than twelve months | 99.00% | 98.00%
Unrealized Loss - Twelve months or more | $ (9) | $ (17)
Fair Value - Twelve months or more | $ 299 | $ 634
Fair Value as % of Cost - Twelve months or more | 97.00% | 97.00%
Corporate and other |  |
Available-for-sale securities in a continuous unrealized loss position |  |
Unrealized Loss - Less than twelve months | $ (20) | $ (93)
Fair Value - Less than twelve months | $ 2,031 | $ 3,849
Fair Value as % of Cost - Less than twelve months | 99.00% | 98.00%
Unrealized Loss - Twelve months or more | $ (18) | $ (19)
Fair Value - Twelve months or more | $ 603 | $ 312
Fair Value as % of Cost - Twelve months or more | 97.00% | 94.00%
Equity securities |  |
Available-for-sale securities in a continuous unrealized loss position |  |
Unrealized Loss - Less than twelve months | $ (22) | $ (29)
Fair Value - Less than twelve months | $ 158 | $ 350
Fair Value as % of Cost - Less than twelve months | 88.00% | 92.00%
Unrealized Loss - Twelve months or more | $ (1) | $ (1)
Fair Value - Twelve months or more | $ 13 | $ 6
Fair Value as % of Cost - Twelve months or more | 93.00% | 86.00%
Common stocks |  |
Available-for-sale securities in a continuous unrealized loss position |  |
Unrealized Loss - Less than twelve months | $ (22) | $ (23)
Fair Value - Less than twelve months | $ 117 | $ 215
Fair Value as % of Cost - Less than twelve months | 84.00% | 90.00%
Unrealized Loss - Twelve months or more | $ 0 | $ 0
Fair Value - Twelve months or more | $ 0 | $ 0
Fair Value as % of Cost - Twelve months or more | 0.00% | 0.00%
Perpetual preferred stocks |  |
Available-for-sale securities in a continuous unrealized loss position |  |
Unrealized Loss - Less than twelve months | $ 0 | $ (6)
Fair Value - Less than twelve months | $ 41 | $ 135
Fair Value as % of Cost - Less than twelve months | 100.00% | 96.00%
Unrealized Loss - Twelve months or more | $ (1) | $ (1)
Fair Value - Twelve months or more | $ 13 | $ 6
Fair Value as % of Cost - Twelve months or more | 93.00% | 86.00%

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