| **Business Segments - Income Statement Information (Details) - USD ($) $ in Millions** | 3 Months Ended | 6 Months Ended |
|----------------------------------------------------------------------------------------|----------------|----------------|
||
| **Operating revenues**                                                                 |           |           |   |   |
| Net earned premiums                                                                    | $ 1,734        | $ 1,730        | $ 3,379 | $ 3,429 |
| Net investment income                                                                  | 475            | 502            | 1,020 | 937 |
| Other revenues                                                                         | 107            | 100            | 211 | 197 |
| Total operating revenues                                                               | 2,316          | 2,332          | 4,610 | 4,563 |
| **Claims, Benefits and Expenses**                                                      |           |           |   |   |
| Net incurred claims and benefits                                                       | 1,275          | 1,335          | 2,564 | 2,739 |
| Policyholders’ dividends                                                         | 5              | 4              | 9 | 8 |
| Amortization of deferred acquisition costs                                             | 312            | 305            | 617 | 612 |
| Other insurance related expenses                                                       | 261            | 270            | 515 | 547 |
| Other expenses                                                                         | 143            | 146            | 278 | 292 |
| Total claims, benefits and expenses                                                    | 1,996          | 2,060          | 3,983 | 4,198 |
| Operating income (loss) before income tax                                              | 320            | 272            | 627 | 365 |
| Income tax (expense) benefit on operating income (loss)                                | (81)           | (71)           | (153) | (73) |
| Net operating income (loss)                                                            | 239            | 201            | 474 | 292 |
| Net realized investment gains (losses)                                                 | 50             | 16             | 86 | (20) |
| Income tax (expense) benefit on net realized investment gains (losses)                 | (17)           | (8)            | (28) | 3 |
| Net realized investment gains (losses), after tax                                      | 33             | 8              | 58 | (17) |
| Net income (loss)                                                                      | 272            | 209            | 532 | 275 |
| Operating Segments | Specialty                                                         |           |           |   |   |
| **Operating revenues**                                                                 |           |           |   |   |
| Net earned premiums                                                                    | 689            | 702            | 1,353 | 1,384 |
| Net investment income                                                                  | 120            | 133            | 273 | 240 |
| Other revenues                                                                         | 98             | 89             | 192 | 176 |
| Total operating revenues                                                               | 907            | 924            | 1,818 | 1,800 |
| **Claims, Benefits and Expenses**                                                      |           |           |   |   |
| Net incurred claims and benefits                                                       | 398            | 377            | 784 | 767 |
| Policyholders’ dividends                                                         | 1              | 1              | 2 | 2 |
| Amortization of deferred acquisition costs                                             | 149            | 148            | 292 | 292 |
| Other insurance related expenses                                                       | 72             | 73             | 141 | 148 |
| Other expenses                                                                         | 82             | 79             | 163 | 154 |
| Total claims, benefits and expenses                                                    | 702            | 678            | 1,382 | 1,363 |
| Operating income (loss) before income tax                                              | 205            | 246            | 436 | 437 |
| Income tax (expense) benefit on operating income (loss)                                | (69)           | (82)           | (146) | (146) |
| Net operating income (loss)                                                            | 136            | 164            | 290 | 291 |
| Net realized investment gains (losses)                                                 | 14             | 4              | 21 | (7) |
| Income tax (expense) benefit on net realized investment gains (losses)                 | (5)            | (2)            | (8) | 2 |
| Net realized investment gains (losses), after tax                                      | 9              | 2              | 13 | (5) |
| Net income (loss)                                                                      | 145            | 166            | 303 | 286 |
| Operating Segments | Commercial                                                        |           |           |   |   |
| **Operating revenues**                                                                 |           |           |   |   |
| Net earned premiums                                                                    | 705            | 696            | 1,356 | 1,384 |
| Net investment income                                                                  | 143            | 164            | 321 | 290 |
| Other revenues                                                                         | 9              | 8              | 18 | 14 |
| Total operating revenues                                                               | 857            | 868            | 1,695 | 1,688 |
| **Claims, Benefits and Expenses**                                                      |           |           |   |   |
| Net incurred claims and benefits                                                       | 422            | 469            | 859 | 911 |
| Policyholders’ dividends                                                         | 4              | 3              | 7 | 6 |
| Amortization of deferred acquisition costs                                             | 118            | 117            | 234 | 233 |
| Other insurance related expenses                                                       | 128            | 130            | 254 | 271 |
| Other expenses                                                                         | 11             | 11             | 25 | 16 |
| Total claims, benefits and expenses                                                    | 683            | 730            | 1,379 | 1,437 |
| Operating income (loss) before income tax                                              | 174            | 138            | 316 | 251 |
| Income tax (expense) benefit on operating income (loss)                                | (59)           | (46)           | (107) | (85) |
| Net operating income (loss)                                                            | 115            | 92             | 209 | 166 |
| Net realized investment gains (losses)                                                 | 19             | 8              | 30 | (10) |
| Income tax (expense) benefit on net realized investment gains (losses)                 | (6)            | (3)            | (9) | 3 |
| Net realized investment gains (losses), after tax                                      | 13             | 5              | 21 | (7) |
| Net income (loss)                                                                      | 128            | 97             | 230 | 159 |
| Operating Segments | International                                                     |           |           |   |   |
| **Operating revenues**                                                                 |           |           |   |   |
| Net earned premiums                                                                    | 206            | 197            | 403 | 395 |
| Net investment income                                                                  | 13             | 13             | 25 | 25 |
| Other revenues                                                                         | (1)            | 0              | (1) | 1 |
| Total operating revenues                                                               | 218            | 210            | 427 | 421 |
| **Claims, Benefits and Expenses**                                                      |           |           |   |   |
| Net incurred claims and benefits                                                       | 129            | 157            | 244 | 278 |
| Policyholders’ dividends                                                         | 0              | 0              | 0 | 0 |
| Amortization of deferred acquisition costs                                             | 45             | 40             | 91 | 87 |
| Other insurance related expenses                                                       | 31             | 37             | 58 | 65 |
| Other expenses                                                                         | (1)            | 7              | (7) | 16 |
| Total claims, benefits and expenses                                                    | 204            | 241            | 386 | 446 |
| Operating income (loss) before income tax                                              | 14             | (31)           | 41 | (25) |
| Income tax (expense) benefit on operating income (loss)                                | (4)            | 4              | (11) | 4 |
| Net operating income (loss)                                                            | 10             | (27)           | 30 | (21) |
| Net realized investment gains (losses)                                                 | 7              | 4              | 13 | 8 |
| Income tax (expense) benefit on net realized investment gains (losses)                 | (1)            | (1)            | (2) | (2) |
| Net realized investment gains (losses), after tax                                      | 6              | 3              | 11 | 6 |
| Net income (loss)                                                                      | 16             | (24)           | 41 | (15) |
| Operating Segments | Life & Group Non-Core                                         |           |           |   |   |
| **Operating revenues**                                                                 |           |           |   |   |
| Net earned premiums                                                                    | 135            | 136            | 268 | 267 |
| Net investment income                                                                  | 195            | 188            | 392 | 375 |
| Other revenues                                                                         | 0              | 3              | 1 | 3 |
| Total operating revenues                                                               | 330            | 327            | 661 | 645 |
| **Claims, Benefits and Expenses**                                                      |           |           |   |   |
| Net incurred claims and benefits                                                       | 328            | 340            | 658 | 663 |
| Policyholders’ dividends                                                         | 0              | 0              | 0 | 0 |
| Amortization of deferred acquisition costs                                             | 0              | 0              | 0 | 0 |
| Other insurance related expenses                                                       | 32             | 31             | 64 | 64 |
| Other expenses                                                                         | 1              | 2              | 3 | 5 |
| Total claims, benefits and expenses                                                    | 361            | 373            | 725 | 732 |
| Operating income (loss) before income tax                                              | (31)           | (46)           | (64) | (87) |
| Income tax (expense) benefit on operating income (loss)                                | 36             | 42             | 73 | 81 |
| Net operating income (loss)                                                            | 5              | (4)            | 9 | (6) |
| Net realized investment gains (losses)                                                 | 7              | (2)            | 17 | (5) |
| Income tax (expense) benefit on net realized investment gains (losses)                 | (3)            | (3)            | (7) | (3) |
| Net realized investment gains (losses), after tax                                      | 4              | (5)            | 10 | (8) |
| Net income (loss)                                                                      | 9              | (9)            | 19 | (14) |
| Operating Segments | Corporate & Other Non-Core                                    |           |           |   |   |
| **Operating revenues**                                                                 |           |           |   |   |
| Net earned premiums                                                                    | 0              | 0              | 0 | 0 |
| Net investment income                                                                  | 4              | 4              | 9 | 7 |
| Other revenues                                                                         | 1              | 0              | 1 | 3 |
| Total operating revenues                                                               | 5              | 4              | 10 | 10 |
| **Claims, Benefits and Expenses**                                                      |           |           |   |   |
| Net incurred claims and benefits                                                       | (2)            | (8)            | 19 | 120 |
| Policyholders’ dividends                                                         | 0              | 0              | 0 | 0 |
| Amortization of deferred acquisition costs                                             | 0              | 0              | 0 | 0 |
| Other insurance related expenses                                                       | (1)            | 0              | (1) | 0 |
| Other expenses                                                                         | 50             | 47             | 94 | 101 |
| Total claims, benefits and expenses                                                    | 47             | 39             | 112 | 221 |
| Operating income (loss) before income tax                                              | (42)           | (35)           | (102) | (211) |
| Income tax (expense) benefit on operating income (loss)                                | 15             | 11             | 38 | 73 |
| Net operating income (loss)                                                            | (27)           | (24)           | (64) | (138) |
| Net realized investment gains (losses)                                                 | 3              | 2              | 5 | (6) |
| Income tax (expense) benefit on net realized investment gains (losses)                 | (2)            | 1              | (2) | 3 |
| Net realized investment gains (losses), after tax                                      | 1              | 3              | 3 | (3) |
| Net income (loss)                                                                      | (26)           | (21)           | (61) | (141) |
| Intersegment Eliminations                                                              |           |           |   |   |
| **Operating revenues**                                                                 |           |           |   |   |
| Net earned premiums                                                                    | (1)            | (1)            | (1) | (1) |
| Net investment income                                                                  | 0              | 0              | 0 | 0 |
| Other revenues                                                                         | 0              | 0              | 0 | 0 |
| Total operating revenues                                                               | (1)            | (1)            | (1) | (1) |
| **Claims, Benefits and Expenses**                                                      |           |           |   |   |
| Net incurred claims and benefits                                                       | 0              | 0              | 0 | 0 |
| Policyholders’ dividends                                                         | 0              | 0              | 0 | 0 |
| Amortization of deferred acquisition costs                                             | 0              | 0              | 0 | 0 |
| Other insurance related expenses                                                       | (1)            | (1)            | (1) | (1) |
| Other expenses                                                                         | 0              | 0              | 0 | 0 |
| Total claims, benefits and expenses                                                    | (1)            | (1)            | (1) | (1) |
| Operating income (loss) before income tax                                              | 0              | 0              | 0 | 0 |
| Income tax (expense) benefit on operating income (loss)                                | 0              | 0              | 0 | 0 |
| Net operating income (loss)                                                            | 0              | 0              | 0 | 0 |
| Net realized investment gains (losses)                                                 | 0              | 0              | 0 | 0 |
| Income tax (expense) benefit on net realized investment gains (losses)                 | 0              | 0              | 0 | 0 |
| Net realized investment gains (losses), after tax                                      | 0              | 0              | 0 | 0 |
| Net income (loss)                                                                      | $ 0            | $ 0            | $ 0 | $ 0 |

| X                                                                                                                                                                                                                                                                                                                  |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Income Tax Expense (Benefit) on Net Realized Investment Gains Losses
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Income Tax Expense (Benefit) on Operating Income (Loss)
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| - Definition
The income earned from investments in securities and property, equipment and other capital assets. It includes rent from property and equipment, dividends from shares in corporations, and interest from bonds, loans, mortgages, derivatives, commercial paper, bank accounts, certificates of deposits, treasuries, and other financial securities. It also includes realized gains and losses on trading securities.
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| - Definition
Net Operating Income (Loss) Attributable to Parent
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| - Definition
Net realized investment gains (losses)
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Operating Income (Loss) before Income Tax
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| - Definition
Operating Revenues
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| - Definition
Other Insurance Related Expenses
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|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
This item represents the difference between the gross realized gains and losses realized on the sale of debt or equity securities categorized neither as held-to-maturity nor trading securities. Additionally, this item would include any losses recognized for other than temporary impairments (OTTI) of the subject investments in debt and equity securities.
+ References
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|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The total amount of expense recognized during the period for future policy benefits, claims and claims adjustment costs, and for selling, general and administrative costs.
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| - Definition
Amount of amortization expense (reversal of expense) for deferred policy acquisition costs.
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| - Definition
Amount after tax of income (loss) from continuing operations attributable to the parent.
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| - Definition
Amount of expense classified as other.
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| - Definition
Amount of revenue and income classified as other.
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| - Definition
Amount, after effects of policies assumed or ceded, of expense related to the provision for policy benefits and costs incurred.
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| - Definition
Amount of expense for dividends to policyholders.
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| - Definition
Amount, after premiums ceded to other entities and premiums assumed by the entity, of premiums earned.
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| X                                                                                                                                                             |
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| X                                                                                                                                                            |
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