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Debt (Schedule of Debt Outstanding) (Details) € in Millions, $ in Millions | Dec. 31, 2018 USD ($) | Sep. 30, 2018 USD ($) | Jun. 30, 2018 USD ($) | Mar. 31, 2018 USD ($) | Mar. 31, 2018 EUR (€) | Dec. 31, 2017 USD ($) | Dec. 31, 2017 EUR (€) | Mar. 31, 2000 USD ($)
--- | --- | --- | --- | --- | --- | --- | --- | ---
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Repurchase agreements | $ 1,418 |  |  |  |  | $ 1,408 |  |
Short-term debt | 509 |  |  |  |  | 1,013 |  |
Long-term debt | 12,087 |  |  |  |  | 11,556 |  |
Trust preferred securities | 308 |  |  |  |  | 308 |  |
Junior Subordinated Debt [Member] | Chubb INA Capital Securities Due 2067 [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Long-term debt | 0 |  |  |  |  | 964 |  |
Debt Instrument, Face Amount |  |  | $ 1,000 |  |  |  |  |
Long-term debt stated interest rate |  |  | 6.375% |  |  |  |  |
Senior Notes |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Long-term debt | 12,087 |  |  |  |  | 11,556 |  |
Senior Notes | INA Senior Notes Due March 2018 [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Debt Instrument, Face Amount |  |  |  | $ 300 |  |  |  |
Long-term debt stated interest rate |  |  |  | 5.80% | 5.80% |  |  |
Senior Notes | INA Senior Notes Due May 2018 [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Debt Instrument, Face Amount |  |  | $ 600 |  |  |  |  |
Long-term debt stated interest rate |  |  | 5.75% |  |  |  |  |
Senior Notes | INA Senior Note Due August 2018 [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Debt Instrument, Face Amount |  | $ 100 |  |  |  |  |  |
Long-term debt stated interest rate |  | 6.60% |  |  |  |  |  |
Senior Notes | INA Senior Notes Due June 2019 [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Long-term debt | 0 |  |  |  |  | 499 |  |
Debt Instrument, Face Amount | $ 500 |  |  |  |  |  |  |
Long-term debt stated interest rate | 5.90% |  |  |  |  |  |  |
Make Whole Premium Additional Percent | 0.40% |  |  |  |  |  |  |
Senior Notes | INA Senior Notes Due November 2020 [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Long-term debt | $ 1,297 |  |  |  |  | 1,296 |  |
Debt Instrument, Face Amount | $ 1,300 |  |  |  |  |  |  |
Long-term debt stated interest rate | 2.30% |  |  |  |  |  |  |
Make Whole Premium Additional Percent | 0.15% |  |  |  |  |  |  |
Senior Notes | INA Senior Notes Due November 2022 [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Long-term debt | $ 996 |  |  |  |  | 995 |  |
Debt Instrument, Face Amount | $ 1,000 |  |  |  |  |  |  |
Long-term debt stated interest rate | 2.875% |  |  |  |  |  |  |
Make Whole Premium Additional Percent | 0.20% |  |  |  |  |  |  |
Senior Notes | INA Senior Notes Due March 2023 [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Long-term debt | $ 473 |  |  |  |  | 472 |  |
Debt Instrument, Face Amount | $ 475 |  |  |  |  |  |  |
Long-term debt stated interest rate | 2.70% |  |  |  |  |  |  |
Make Whole Premium Additional Percent | 0.10% |  |  |  |  |  |  |
Senior Notes | INA Senior Notes Due May 2024 [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Long-term debt | $ 696 |  |  |  |  | 695 |  |
Debt Instrument, Face Amount | $ 700 |  |  |  |  |  |  |
Long-term debt stated interest rate | 3.35% |  |  |  |  |  |  |
Make Whole Premium Additional Percent | 0.15% |  |  |  |  |  |  |
Senior Notes | INA Senior Notes Due March 2025 [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Long-term debt | $ 796 |  |  |  |  | 795 |  |
Debt Instrument, Face Amount | $ 800 |  |  |  |  |  |  |
Long-term debt stated interest rate | 3.15% |  |  |  |  |  |  |
Make Whole Premium Additional Percent | 0.15% |  |  |  |  |  |  |
Senior Notes | INA Senior Notes Due May 2026 [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Long-term debt | $ 1,491 |  |  |  |  | 1,489 |  |
Debt Instrument, Face Amount | $ 1,500 |  |  |  |  |  |  |
Long-term debt stated interest rate | 3.35% |  |  |  |  |  |  |
Make Whole Premium Additional Percent | 0.20% |  |  |  |  |  |  |
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% |  |  |  |  |  |  |
Senior Notes | Chubb INA Senior Notes Due 2031 [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Long-term debt | $ 250 |  |  |  |  | 254 |  |
Debt Instrument, Face Amount | $ 200 |  |  |  |  |  |  |
Long-term debt stated interest rate | 6.80% |  |  |  |  |  |  |
Make Whole Premium Additional Percent | 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% |  |  |  |  |  |  |
Make Whole Premium Additional Percent | 0.20% |  |  |  |  |  |  |
Senior Notes | Chubb INA Senior Notes Due 2037 [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Long-term debt | $ 962 |  |  |  |  | 971 |  |
Debt Instrument, Face Amount | $ 800 |  |  |  |  |  |  |
Long-term debt stated interest rate | 6.00% |  |  |  |  |  |  |
Make Whole Premium Additional Percent | 0.20% |  |  |  |  |  |  |
Senior Notes | Chubb INA Senior Notes Due 2038 [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Long-term debt | $ 759 |  |  |  |  | 768 |  |
Debt Instrument, Face Amount | $ 600 |  |  |  |  |  |  |
Long-term debt stated interest rate | 6.50% |  |  |  |  |  |  |
Make Whole Premium Additional Percent | 0.30% |  |  |  |  |  |  |
Senior Notes | INA Senior Note Due March 2028 [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Long-term debt | $ 1,008 |  |  |  |  | 0 | € 0 |
Debt Instrument, Face Amount |  |  |  | $ 1,100 | € 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 March 2043 [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Long-term debt | 470 |  |  |  |  | 469 |  |
Debt Instrument, Face Amount | $ 475 |  |  |  |  |  |  |
Long-term debt stated interest rate | 4.15% |  |  |  |  |  |  |
Make Whole Premium Additional Percent | 0.15% |  |  |  |  |  |  |
Senior Notes | INA Senior Note Due March 2038 [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Long-term debt | $ 1,008 |  |  |  |  | 0 | € 0 |
Debt Instrument, Face Amount |  |  |  | $ 1,100 | € 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 November 2045 [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Long-term debt | 1,483 |  |  |  |  | 1,482 |  |
Debt Instrument, Face Amount | $ 1,500 |  |  |  |  |  |  |
Long-term debt stated interest rate | 4.35% |  |  |  |  |  |  |
Make Whole Premium Additional Percent | 0.25% |  |  |  |  |  |  |
Senior Notes | Other |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Long-term debt | $ 1 |  |  |  |  | 10 |  |
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%
Repurchase agreements |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Short-term debt | $ 1,418 |  |  |  |  | $ 1,408 |  |
Weighted average interest rate on short-term debt | 2.50% |  |  |  |  | 1.50% | 1.50% |
Senior Notes | INA Senior Notes Due March 2018 [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Short-term debt | $ 0 |  |  |  |  | $ 300 |  |
Debt Instrument, Face Amount | $ 300 |  |  |  |  |  |  |
Long-term debt stated interest rate | 5.80% |  |  |  |  |  |  |
Make Whole Premium Additional Percent | 0.35% |  |  |  |  |  |  |
Senior Notes | INA Senior Notes Due May 2018 [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Short-term debt | $ 0 |  |  |  |  | 610 |  |
Debt Instrument, Face Amount | $ 600 |  |  |  |  |  |  |
Long-term debt stated interest rate | 5.75% |  |  |  |  |  |  |
Make Whole Premium Additional Percent | 0.30% |  |  |  |  |  |  |
Senior Notes | INA Senior Note Due August 2018 [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Short-term debt | $ 0 |  |  |  |  | 103 |  |
Debt Instrument, Face Amount | $ 100 |  |  |  |  |  |  |
Long-term debt stated interest rate | 6.60% |  |  |  |  |  |  |
Make Whole Premium Additional Percent | 0.00% |  |  |  |  |  |  |
Senior Notes | INA Senior Note Due June 2019 [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Short-term debt | $ 500 |  |  |  |  | 0 |  |
Debt Instrument, Face Amount | $ 500 |  |  |  |  |  |  |
Long-term debt stated interest rate | 5.90% |  |  |  |  |  |  |
Make Whole Premium Additional Percent | 0.40% |  |  |  |  |  |  |
Senior Notes | INA Senior Note Due December 2019 [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Short-term debt | $ 9 |  |  |  |  | $ 0 |  |
Long-term debt stated interest rate | 7.10% |  |  |  |  |  |  |
Minimum | Junior Subordinated Debt [Member] | Chubb INA Capital Securities Due 2067 [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Make Whole Premium Additional Percent | 0.25% |  |  |  |  |  |  |
Minimum | Senior Notes | Other |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Long-term debt stated interest rate | 2.75% |  |  |  |  |  |  |
Maximum | Junior Subordinated Debt [Member] | Chubb INA Capital Securities Due 2067 [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Make Whole Premium Additional Percent | 0.50% |  |  |  |  |  |  |
Maximum | Senior Notes | Other |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Long-term debt stated interest rate | 7.10% |  |  |  |  |  |  |

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