| **Investments - Summary of fixed maturity and equity securities (Details) - USD ($) $ in Millions** | Jun. 30, 2017 | Dec. 31, 2016 |
|-----------------------------------------------------------------------------------------------------|---------------|---------------|
| **Schedule of Available-for-Sale and Trading Securities [Line Items]**                              |          |          |
| Cost or Amortized Cost                                                                              | $ 38,776      | $ 38,467      |
| Gross Unrealized Gains                                                                              | 3,196         | 2,744         |
| Gross Unrealized Losses                                                                             | 105           | 196           |
| Estimated Fair Value                                                                                | 41,867        | 41,015        |
| Corporate and other bonds                                                                           |          |          |
| **Schedule of Available-for-Sale and Trading Securities [Line Items]**                              |          |          |
| Cost or Amortized Cost                                                                              | 17,823        | 17,711        |
| Gross Unrealized Gains                                                                              | 1,589         | 1,323         |
| Gross Unrealized Losses                                                                             | 29            | 76            |
| Estimated Fair Value                                                                                | 19,383        | 18,958        |
| Unrealized OTTI Losses (Gains)                                                                      | 0             | (1)           |
| States, municipalities and political subdivisions                                                   |          |          |
| **Schedule of Available-for-Sale and Trading Securities [Line Items]**                              |          |          |
| Cost or Amortized Cost                                                                              | 12,461        | 12,060        |
| Gross Unrealized Gains                                                                              | 1,380         | 1,213         |
| Gross Unrealized Losses                                                                             | 15            | 33            |
| Estimated Fair Value                                                                                | 13,826        | 13,240        |
| Unrealized OTTI Losses (Gains)                                                                      | (13)          | (16)          |
| Residential mortgage-backed                                                                         |          |          |
| **Schedule of Available-for-Sale and Trading Securities [Line Items]**                              |          |          |
| Cost or Amortized Cost                                                                              | 4,835         | 5,004         |
| Gross Unrealized Gains                                                                              | 124           | 120           |
| Gross Unrealized Losses                                                                             | 38            | 51            |
| Estimated Fair Value                                                                                | 4,921         | 5,073         |
| Unrealized OTTI Losses (Gains)                                                                      | (27)          | (28)          |
| Commercial mortgage-backed                                                                          |          |          |
| **Schedule of Available-for-Sale and Trading Securities [Line Items]**                              |          |          |
| Cost or Amortized Cost                                                                              | 1,907         | 2,016         |
| Gross Unrealized Gains                                                                              | 59            | 48            |
| Gross Unrealized Losses                                                                             | 14            | 24            |
| Estimated Fair Value                                                                                | 1,952         | 2,040         |
| Unrealized OTTI Losses (Gains)                                                                      | 0             | 0             |
| Other asset-backed                                                                                  |          |          |
| **Schedule of Available-for-Sale and Trading Securities [Line Items]**                              |          |          |
| Cost or Amortized Cost                                                                              | 1,050         | 1,022         |
| Gross Unrealized Gains                                                                              | 16            | 8             |
| Gross Unrealized Losses                                                                             | 5             | 5             |
| Estimated Fair Value                                                                                | 1,061         | 1,025         |
| Unrealized OTTI Losses (Gains)                                                                      | 0             | 0             |
| Total asset-backed                                                                                  |          |          |
| **Schedule of Available-for-Sale and Trading Securities [Line Items]**                              |          |          |
| Cost or Amortized Cost                                                                              | 7,792         | 8,042         |
| Gross Unrealized Gains                                                                              | 199           | 176           |
| Gross Unrealized Losses                                                                             | 57            | 80            |
| Estimated Fair Value                                                                                | 7,934         | 8,138         |
| Unrealized OTTI Losses (Gains)                                                                      | (27)          | (28)          |
| U.S. Treasury and obligations of government-sponsored enterprises                                   |          |          |
| **Schedule of Available-for-Sale and Trading Securities [Line Items]**                              |          |          |
| Cost or Amortized Cost                                                                              | 113           | 83            |
| Gross Unrealized Gains                                                                              | 4             | 10            |
| Gross Unrealized Losses                                                                             | 2             | 0             |
| Estimated Fair Value                                                                                | 115           | 93            |
| Unrealized OTTI Losses (Gains)                                                                      | 0             | 0             |
| Foreign government                                                                                  |          |          |
| **Schedule of Available-for-Sale and Trading Securities [Line Items]**                              |          |          |
| Cost or Amortized Cost                                                                              | 438           | 435           |
| Gross Unrealized Gains                                                                              | 12            | 13            |
| Gross Unrealized Losses                                                                             | 1             | 3             |
| Estimated Fair Value                                                                                | 449           | 445           |
| Unrealized OTTI Losses (Gains)                                                                      | 0             | 0             |
| Redeemable preferred stock                                                                          |          |          |
| **Schedule of Available-for-Sale and Trading Securities [Line Items]**                              |          |          |
| Cost or Amortized Cost                                                                              | 18            | 18            |
| Gross Unrealized Gains                                                                              | 1             | 1             |
| Gross Unrealized Losses                                                                             | 0             | 0             |
| Estimated Fair Value                                                                                | 19            | 19            |
| Unrealized OTTI Losses (Gains)                                                                      | 0             | 0             |
| Total fixed maturity securities available-for-sale                                                  |          |          |
| **Schedule of Available-for-Sale and Trading Securities [Line Items]**                              |          |          |
| Cost or Amortized Cost                                                                              | 38,645        | 38,349        |
| Gross Unrealized Gains                                                                              | 3,185         | 2,736         |
| Gross Unrealized Losses                                                                             | 104           | 192           |
| Estimated Fair Value                                                                                | 41,726        | 40,893        |
| Unrealized OTTI Losses (Gains)                                                                      | (40)          | (45)          |
| Total fixed maturity securities trading                                                             |          |          |
| **Schedule of Available-for-Sale and Trading Securities [Line Items]**                              |          |          |
| Total fixed maturity securities trading, cost                                                       | 23            | 12            |
| Total fixed maturity securities trading, fair value                                                 | 23            | 12            |
| Equity securities available-for-sale -- Common stock                                                |          |          |
| **Schedule of Available-for-Sale and Trading Securities [Line Items]**                              |          |          |
| Cost or Amortized Cost                                                                              | 17            | 13            |
| Gross Unrealized Gains                                                                              | 5             | 6             |
| Gross Unrealized Losses                                                                             | 0             | 0             |
| Estimated Fair Value                                                                                | 22            | 19            |
| Preferred stock                                                                                     |          |          |
| **Schedule of Available-for-Sale and Trading Securities [Line Items]**                              |          |          |
| Cost or Amortized Cost                                                                              | 91            | 93            |
| Gross Unrealized Gains                                                                              | 6             | 2             |
| Gross Unrealized Losses                                                                             | 1             | 4             |
| Estimated Fair Value                                                                                | 96            | 91            |
| Equity securities                                                                                   |          |          |
| **Schedule of Available-for-Sale and Trading Securities [Line Items]**                              |          |          |
| Cost or Amortized Cost                                                                              | 108           | 106           |
| Gross Unrealized Gains                                                                              | 11            | 8             |
| Gross Unrealized Losses                                                                             | 1             | 4             |
| Estimated Fair Value                                                                                | $ 118         | $ 110         |

| X                                                                                                                                                                                                                                                                                                                                                                                                |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The accumulated unrealized gain or loss position on Other Than Temporary Impairment (OTTI) Securities
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|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.
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|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.
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| - Definition
Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.
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| - Definition
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| - Definition
Amount of investments in debt and equity securities and other forms of securities that provide ownership interests classified as trading.
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Cost of investments in debt and equity securities and other forms of securities that provide ownership interests classified as trading.
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