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Quarterly Operating Results (Unaudited) - Narrative (Details) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2018 | Sep. 30, 2018 | Jun. 30, 2018 | Dec. 31, 2017 | Sep. 30, 2017 | Jun. 30, 2017 | Mar. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Quarterly Operating Results (Unaudited) [Line Items] |  |  |  |  |  |  |  |  |  |  |
S&P 500 Index increase (decrease) during period | (14.00%) |  |  |  |  |  |  |  |  |  |
Pretax catastrophe losses |  |  |  |  | $ 107 |  |  |  | $ 103 | $ 140 |
Income tax expense (benefit) |  |  |  |  |  |  |  |  | 122 | 247 | $ 119
Property and casualty insurance | Other lines |  |  |  |  |  |  |  |  |  |  |
Quarterly Operating Results (Unaudited) [Line Items] |  |  |  |  |  |  |  |  |  |  |
Special charges to increase asbestos and environmental reserves |  | $ 18 |  |  | 89 |  |  |  | 18 | 89 | 36
Favorable reserve development related to certain lines of business within subsidiaries in connection with a reinsurance to close agreement |  |  |  |  |  |  |  |  |  | 18 |
Property and casualty insurance | Real estate investment |  |  |  |  |  |  |  |  |  |  |
Quarterly Operating Results (Unaudited) [Line Items] |  |  |  |  |  |  |  |  |  |  |
Pretax income from the sale of real estate properties included in other income |  |  |  |  |  |  | $ 13 |  |  |  | 32
Annuity |  |  |  |  |  |  |  |  |  |  |
Quarterly Operating Results (Unaudited) [Line Items] |  |  |  |  |  |  |  |  |  |  |
S&P 500 impact on annuity earnings before income taxes beyond the impact of derivatives | $ (30) |  |  |  |  |  |  |  |  |  |
Other |  |  |  |  |  |  |  |  |  |  |
Quarterly Operating Results (Unaudited) [Line Items] |  |  |  |  |  |  |  |  |  |  |
Special charges to increase asbestos and environmental reserves |  |  |  |  |  |  |  |  | 9 | 24 | 5
Write off of deferred debt issuance cost due to the retirement of debt |  |  |  |  | 4 | $ 7 |  |  |  |  |
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 |  | $ 9 |  |  | $ 24 |  |  |  |  |  |
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 |  |  |  |  |  |  |
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) |  |  |  |  | 122 | $ 247 | $ 119
Annuity benefits | Annuity |  |  |  |  |  |  |  |  |  |  |
Quarterly Operating Results (Unaudited) [Line Items] |  |  |  |  |  |  |  |  |  |  |
Charges resulting from updating (unlocking) actuarial assumptions | 4 |  | $ 27 | $ 3 |  |  |  |  |  |  |
Equity securities | Accounting Standards Update 2016-01 | Equity securities still owned |  |  |  |  |  |  |  |  |  |  |
Quarterly Operating Results (Unaudited) [Line Items] |  |  |  |  |  |  |  |  |  |  |
Net holding gains (losses) on equity securities |  |  |  |  |  |  |  |  | (257) |  |
Equity securities | Accounting Standards Update 2016-01 | Equity securities still owned | Realized gains (losses) on securities |  |  |  |  |  |  |  |  |  |  |
Quarterly Operating Results (Unaudited) [Line Items] |  |  |  |  |  |  |  |  |  |  |
Net holding gains (losses) on equity securities | $ (228) |  |  |  |  |  |  |  | $ (279) |  |

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