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Insurance - Narrative (Details) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2017 | Sep. 30, 2017 | Sep. 30, 2016 | Jun. 30, 2016 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Insurance [Line Items] |  |  |  |  |  |  |
Carrying value of cash and securities owned by U.S.-based insurance subsidiaries on deposit | $ 1,080 |  |  |  | $ 1,080 |  |
Carrying value of cash and securities owned by insurance subsidiaries on deposit in support of underwriting activities at Lloyd's | 205 |  |  |  | 205 |  |
Undrawn letters of credit | 447 |  |  |  | 447 |  |
Undrawn letters of credit, collateralized | 0 |  |  |  | $ 0 |  |
Workers' compensation insurance discount rate |  |  |  |  | 4.50% | 4.50% |
Workers' compensation insurance discount, which has reduced the liability for unpaid losses and loss adjustment expenses | 15 |  |  |  | $ 15 | $ 16 |
Long-term care insurance reserves, net of reinsurance recoverables | 39 |  |  |  | 39 | 37 |
Investment in Federal Home Loan Bank capital stock | 44 |  |  |  | 44 |  |
Repayments under funding agreements with the Federal Home Loan Bank | 64 |  |  |  |  |  |
Advances from Federal Home Loan Bank | 871 |  |  |  | $ 871 |  |
Description of variable rate basis |  |  |  |  | LIBOR |  |
Repayments of advances from Federal Home Loan Bank due in next twelve months | 285 |  |  |  | $ 285 |  |
Repayments of advances from Federal Home Loan Bank due in year three | 586 |  |  |  | 586 |  |
Fair value of fixed maturity investments held as collateral by the Federal Home Loan Bank | 1,040 |  |  |  | 1,040 |  |
Interest on advances from Federal Home Loan Bank |  |  |  |  | 14 | 8 | $ 3
Maximum amount of dividends available to be paid by insurance subsidiaries to AFG without prior approval of regulatory authorities | 826 |  |  |  | 826 |  |
Dividends on Common Stock |  |  |  |  | (421) | (187) | (178)
Aggregate guaranteed minimum death benefit value on variable annuity polices in force | 15 |  |  |  | 15 | 20 |
Minimum |  |  |  |  |  |  |
Insurance [Line Items] |  |  |  |  |  |  |
Dividends payments without violating the most restrictive debt covenants, minimum | 1,000 |  |  |  | $ 1,000 |  |
Minimum | LIBOR |  |  |  |  |  |  |
Insurance [Line Items] |  |  |  |  |  |  |
FHLB advances, basis spread on variable rate |  |  |  |  | 0.03% |  |
Maximum | LIBOR |  |  |  |  |  |  |
Insurance [Line Items] |  |  |  |  |  |  |
FHLB advances, basis spread on variable rate |  |  |  |  | 0.35% |  |
Average | LIBOR |  |  |  |  |  |  |
Insurance [Line Items] |  |  |  |  |  |  |
FHLB advances, basis spread on variable rate |  |  |  |  | 1.75% |  |
FHLB, advances, refinanced terms |  |  |  |  |  |  |
Insurance [Line Items] |  |  |  |  |  |  |
Advances from Federal Home Loan Bank | 831 |  |  |  | $ 831 |  |
Catastrophe Bonds |  |  |  |  |  |  |
Insurance [Line Items] |  |  |  |  |  |  |
Catastrophe reinsurance coverage, excess retention percentage |  |  |  |  | 95.00% |  |
Catastrophe reinsurance coverage, excess retention amount |  |  |  |  | $ 200 |  |
Catastrophe reinsurance coverage, retention amount |  |  |  |  | 100 |  |
Catastrophe reinsurance coverage, annual cost |  |  |  |  | 11 |  |
Life insurance |  |  |  |  |  |  |
Insurance [Line Items] |  |  |  |  |  |  |
Life insurance in force, ceded premiums | 8,320 |  |  |  | 8,320 | 10,220 |
Life insurance in force, direct premiums | 12,050 |  |  |  | 12,050 | 13,490 |
Life written premiums ceded |  |  |  |  | 28 | 31 | 40
Reinsurance recoveries |  |  |  |  | 35 | 41 | 50
Variable annuities | Maximum |  |  |  |  |  |  |
Insurance [Line Items] |  |  |  |  |  |  |
Death benefits paid in excess of the variable annuity account balances (are less than in 2014, 2015 and 2016) |  |  |  |  | 1 | 1 | 1
Property and casualty insurance | Other lines |  |  |  |  |  |  |
Insurance [Line Items] |  |  |  |  |  |  |
Favorable reserve development related to certain lines of business within subsidiaries in connection with a reinsurance to close agreement |  |  |  |  | 18 |  |
Special charges to increase asbestos and environmental reserves |  | $ 89 | $ 36 |  | $ 89 | 36 | $ 67
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 |  |  |  |  |  |  |
Insurance [Line Items] |  |  |  |  |  |  |
Special charges related to the exit of certain lines of business within subsidiaries |  |  |  | $ 57 |  |  |
Neon Capital Limited | Property and casualty insurance |  |  |  |  |  |  |
Insurance [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 |  |  |  |  |  |  |
Insurance [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 | Property and casualty insurance | Other lines | Reinsurance to close agreement |  |  |  |  |  |  |
Insurance [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 |  |  |  |  |  |  |
Insurance [Line Items] |  |  |  |  |  |  |
Favorable reserve development related to certain lines of business within subsidiaries in connection with a reinsurance to close agreement | $ 24 |  |  |  |  |  |

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