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Quarterly Operating Results (Unaudited) - Narrative (Details) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2017 | Sep. 30, 2017 | Jun. 30, 2017 | Mar. 31, 2017 | Dec. 31, 2016 | Sep. 30, 2016 | Jun. 30, 2016 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Quarterly Operating Results (Unaudited) [Line Items] |  |  |  |  |  |  |  |  |  |
Pretax catastrophe losses |  | $ 107 |  |  |  |  |  | $ 140 | $ 55 |
Income tax expense (benefit) |  |  |  |  |  |  |  | 247 | 119 | $ 195
Property and casualty insurance | Other lines |  |  |  |  |  |  |  |  |  |
Quarterly Operating Results (Unaudited) [Line Items] |  |  |  |  |  |  |  |  |  |
Special charges to increase asbestos and environmental reserves |  | 89 |  |  |  | $ 36 |  | 89 | 36 | 67
Favorable reserve development related to certain lines of business within subsidiaries in connection with a reinsurance to close agreement |  |  |  |  |  |  |  | 18 |  |
Special charges related to the exit of certain lines of business within subsidiaries |  |  |  |  |  |  | $ 65 |  | 65 |
Property and casualty insurance | Other lines | Increased insurance loss reserves related to medical malpractice and general liability lines |  |  |  |  |  |  |  |  |  |
Quarterly Operating Results (Unaudited) [Line Items] |  |  |  |  |  |  |  |  |  |
Special charges related to the exit of certain lines of business within subsidiaries |  |  |  |  |  |  | 57 |  |  |
Other |  |  |  |  |  |  |  |  |  |
Quarterly Operating Results (Unaudited) [Line Items] |  |  |  |  |  |  |  |  |  |
Special charges to increase asbestos and environmental reserves |  |  |  |  |  |  |  | 24 | 5 | 12
Write off of deferred debt issuance cost due to the retirement of debt |  | 4 | $ 7 |  |  |  |  |  |  | 4
Make-whole premium, settlement of interest rate swap and write off of deferred debt issuance cost included in the loss on the retirement of debt | $ 40 |  |  |  |  |  |  | 51 |  |
Other | Former railroad and manufacturing operations |  |  |  |  |  |  |  |  |  |
Quarterly Operating Results (Unaudited) [Line Items] |  |  |  |  |  |  |  |  |  |
Special charges to increase asbestos and environmental reserves |  | $ 24 |  |  |  | $ 5 |  |  |  |
Neon Capital Limited |  |  |  |  |  |  |  |  |  |
Quarterly Operating Results (Unaudited) [Line Items] |  |  |  |  |  |  |  |  |  |
Income tax expense (benefit) | (56) |  |  |  | $ (111) |  |  |  | (48) |
Neon Capital Limited | Property and casualty insurance |  |  |  |  |  |  |  |  |  |
Quarterly Operating Results (Unaudited) [Line Items] |  |  |  |  |  |  |  |  |  |
Favorable reserve development related to certain lines of business within subsidiaries in connection with a reinsurance to close agreement | 42 |  |  |  |  |  |  |  |  |
Neon Capital Limited | Property and casualty insurance | Other lines |  |  |  |  |  |  |  |  |  |
Quarterly Operating Results (Unaudited) [Line Items] |  |  |  |  |  |  |  |  |  |
Favorable reserve development related to certain lines of business within subsidiaries in connection with a reinsurance to close agreement | 18 |  |  |  |  |  |  |  |  |
Special charges related to the exit of certain lines of business within subsidiaries |  |  |  |  |  |  |  |  | 65 |
Neon Capital Limited | Neon Capital Limited | Current lines |  |  |  |  |  |  |  |  |  |
Quarterly Operating Results (Unaudited) [Line Items] |  |  |  |  |  |  |  |  |  |
Favorable reserve development related to certain lines of business within subsidiaries in connection with a reinsurance to close agreement | 24 |  |  |  |  |  |  |  |  |
AFG |  |  |  |  |  |  |  |  |  |
Quarterly Operating Results (Unaudited) [Line Items] |  |  |  |  |  |  |  |  |  |
Income tax expense (benefit) | $ (56) |  |  |  | $ (177) |  |  | $ 247 | 119 | 195
Real estate investment | Property and casualty insurance |  |  |  |  |  |  |  |  |  |
Quarterly Operating Results (Unaudited) [Line Items] |  |  |  |  |  |  |  |  |  |
Pretax income from the sale of real estate properties included in other income |  |  |  | $ 13 |  |  | $ 32 |  | $ 32 | $ 66

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