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Debt (Schedule of Debt Outstanding) (Details) € in Millions, $ in Millions | Dec. 31, 2019 USD ($) | Dec. 31, 2019 EUR (€) | Jun. 30, 2019 USD ($) | Dec. 31, 2018 USD ($) | Mar. 31, 2000 USD ($)
--- | --- | --- | --- | --- | ---
Debt Instrument [Line Items] |  |  |  |  |
Repurchase agreements | $ 1,416 |  |  | $ 1,418 |
Short-term debt | 1,299 |  |  | 509 |
Long-term debt | 13,559 |  |  | 12,087 |
Trust preferred securities | 308 |  |  | 308 |
Senior Notes |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Long-term debt | 13,559 |  |  | 12,087 |
Senior Notes | INA Senior Notes Due November 2020 [Member] |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Long-term debt | 0 |  |  | 1,297 |
Debt Instrument, Face Amount | $ 1,300 |  |  |  |
Long-term debt stated interest rate | 2.30% | 2.30% |  |  |
Make Whole Premium Additional Percent | 0.15% | 0.15% |  |  |
Senior Notes | INA Senior Notes Due November 2022 [Member] |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Long-term debt | $ 997 |  |  | 996 |
Debt Instrument, Face Amount | $ 1,000 |  |  |  |
Long-term debt stated interest rate | 2.875% | 2.875% |  |  |
Make Whole Premium Additional Percent | 0.20% | 0.20% |  |  |
Senior Notes | INA Senior Notes Due March 2023 [Member] |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Long-term debt | $ 473 |  |  | 473 |
Debt Instrument, Face Amount | $ 475 |  |  |  |
Long-term debt stated interest rate | 2.70% | 2.70% |  |  |
Make Whole Premium Additional Percent | 0.10% | 0.10% |  |  |
Senior Notes | INA Senior Notes Due May 2024 [Member] |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Long-term debt | $ 697 |  |  | 696 |
Debt Instrument, Face Amount | $ 700 |  |  |  |
Long-term debt stated interest rate | 3.35% | 3.35% |  |  |
Make Whole Premium Additional Percent | 0.15% | 0.15% |  |  |
Senior Notes | INA Senior Notes Due December 2024 [Member] |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Long-term debt | $ 776 |  |  | 0 |
Debt Instrument, Face Amount | $ 779 | € 700 |  |  |
Long-term debt stated interest rate | 0.30% | 0.30% |  |  |
Make Whole Premium Additional Percent | 0.15% | 0.15% |  |  |
Senior Notes | INA Senior Notes Due March 2025 [Member] |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Long-term debt | $ 796 |  |  | 796 |
Debt Instrument, Face Amount | $ 800 |  |  |  |
Long-term debt stated interest rate | 3.15% | 3.15% |  |  |
Make Whole Premium Additional Percent | 0.15% | 0.15% |  |  |
Senior Notes | INA Senior Notes Due May 2026 [Member] |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Long-term debt | $ 1,492 |  |  | 1,491 |
Debt Instrument, Face Amount | $ 1,500 |  |  |  |
Long-term debt stated interest rate | 3.35% | 3.35% |  |  |
Make Whole Premium Additional Percent | 0.20% | 0.20% |  |  |
Senior Notes | INA Senior Notes Due June 2027 [Member] |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Long-term debt | $ 635 |  |  | 0 |
Debt Instrument, Face Amount | € |  | € 575 |  |  |
Long-term debt stated interest rate | 0.875% | 0.875% |  |  |
Make Whole Premium Additional Percent | 0.20% | 0.20% |  |  |
Senior Notes | INA Senior Notes Due March 2028 [Member] |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Long-term debt | $ 993 |  |  | 1,008 |
Debt Instrument, Face Amount | € |  | € 900 |  |  |
Long-term debt stated interest rate | 1.55% | 1.55% |  |  |
Make Whole Premium Additional Percent | 0.15% | 0.15% |  |  |
Senior Notes | INA Senior Notes Due August 2029 [Member] |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Long-term debt | $ 100 |  |  | 100 |
Debt Instrument, Face Amount | $ 100 |  |  |  |
Long-term debt stated interest rate | 8.875% | 8.875% |  |  |
Senior Notes | INA Senior Notes Due December 2029 [Member] |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Long-term debt | $ 775 |  |  | 0 |
Debt Instrument, Face Amount | $ 779 | € 700 |  |  |
Long-term debt stated interest rate | 0.875% | 0.875% |  |  |
Make Whole Premium Additional Percent | 0.20% | 0.20% |  |  |
Senior Notes | INA Senior Notes Due June 2031 [Member] |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Long-term debt | $ 633 |  |  | 0 |
Debt Instrument, Face Amount | € |  | € 575 |  |  |
Long-term debt stated interest rate | 1.40% | 1.40% |  |  |
Make Whole Premium Additional Percent | 0.25% | 0.25% |  |  |
Senior Notes | INA Senior Notes Due November 2031 [Member] |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Long-term debt | $ 246 |  |  | 250 |
Debt Instrument, Face Amount | $ 200 |  |  |  |
Long-term debt stated interest rate | 6.80% | 6.80% |  |  |
Make Whole Premium Additional Percent | 0.25% | 0.25% |  |  |
Senior Notes | INA Senior Notes Due May 2036 [Member] |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Long-term debt | $ 297 |  |  | 297 |
Debt Instrument, Face Amount | $ 300 |  |  |  |
Long-term debt stated interest rate | 6.70% | 6.70% |  |  |
Make Whole Premium Additional Percent | 0.20% | 0.20% |  |  |
Senior Notes | INA Senior Notes Due May 2037 [Member] |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Long-term debt | $ 953 |  |  | 962 |
Debt Instrument, Face Amount | $ 800 |  |  |  |
Long-term debt stated interest rate | 6.00% | 6.00% |  |  |
Make Whole Premium Additional Percent | 0.20% | 0.20% |  |  |
Senior Notes | INA Senior Notes Due March 2038 [Member] |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Long-term debt | $ 992 |  |  | 1,008 |
Debt Instrument, Face Amount | € |  | € 900 |  |  |
Long-term debt stated interest rate | 2.50% | 2.50% |  |  |
Make Whole Premium Additional Percent | 0.25% | 0.25% |  |  |
Senior Notes | INA Senior Notes Due May 2038 [Member] |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Long-term debt | $ 751 |  |  | 759 |
Debt Instrument, Face Amount | $ 600 |  |  |  |
Long-term debt stated interest rate | 6.50% | 6.50% |  |  |
Make Whole Premium Additional Percent | 0.30% | 0.30% |  |  |
Senior Notes | INA Senior Notes Due March 2043 [Member] |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Long-term debt | $ 470 |  |  | 470 |
Debt Instrument, Face Amount | $ 475 |  |  |  |
Long-term debt stated interest rate | 4.15% | 4.15% |  |  |
Make Whole Premium Additional Percent | 0.15% | 0.15% |  |  |
Senior Notes | INA Senior Notes Due November 2045 [Member] |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Long-term debt | $ 1,483 |  |  | 1,483 |
Debt Instrument, Face Amount | $ 1,500 |  |  |  |
Long-term debt stated interest rate | 4.35% | 4.35% |  |  |
Make Whole Premium Additional Percent | 0.25% | 0.25% |  |  |
Senior Notes | Other Long Term Debt due September 2020 [Member] |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Long-term debt | $ 0 |  |  | 1 |
Long-term debt stated interest rate | 2.75% | 2.75% |  |  |
Trust Preferred Securities | INA capital securities due 2030 |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Debt Instrument, Face Amount | $ 309 |  |  |  | $ 300
Trust preferred securities | $ 308 |  |  | 308 |
Long-term debt stated interest rate | 9.70% | 9.70% |  |  | 9.70%
Repurchase agreements |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Short-term debt | $ 1,416 |  |  | $ 1,418 |
Weighted average interest rate on short-term debt | 2.20% | 2.20% |  | 2.50% |
Senior Notes | INA Senior Note Due June 2019 [Member] |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Short-term debt | $ 0 |  |  | $ 500 |
Make Whole Premium Additional Percent | 0.40% | 0.40% |  |  |
Senior Notes | INA Senior Notes Due June 2019 [Member] |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Debt Instrument, Face Amount | $ 500 |  | $ 500 |  |
Long-term debt stated interest rate | 5.90% | 5.90% | 5.90% |  |
Senior Notes | INA Senior Notes Due November 2020 [Member] |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Short-term debt | $ 1,298 |  |  | 0 |
Debt Instrument, Face Amount | $ 1,300 |  |  |  |
Long-term debt stated interest rate | 2.30% | 2.30% |  |  |
Make Whole Premium Additional Percent | 0.15% | 0.15% |  |  |
Senior Notes | Other Short Term Debt due September 2020 [Member] |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Short-term debt | $ 1 |  |  |  |
Long-term debt stated interest rate | 2.75% | 2.75% |  |  |
Senior Notes | Other Short Term Debt due December 2019 [Member] |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Short-term debt |  |  |  | $ 9 |
Long-term debt stated interest rate |  |  |  | 7.10% |

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