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Income Taxes - Narrative (Details) £ in Millions, $ in Millions |  | 1 Months Ended | 3 Months Ended | 12 Months Ended |
--- | --- | --- | --- | --- | ---
Jan. 01, 2018 | Jan. 31, 2008 USD ($) | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($) | Jun. 30, 2016 USD ($) | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($) | Dec. 31, 2015 USD ($) | Dec. 31, 2012 USD ($) | Dec. 31, 2017 GBP (£)
Income Tax Reconciliation [Line Items] |  |  |  |  |  |  |  |  |  |  |
Statutory rate of income taxes |  |  |  |  |  |  | 35.00% | 35.00% | 35.00% |  |
Payments to acquire additional interest in subsidiaries |  |  |  |  |  |  | $ 0 | $ 315 | $ 0 |  |
impact of foreign operations included in the provision for income taxes |  |  |  |  |  |  | 21 | 5 | 3 |  |
Decrease in the valuation allowance related to the change in the U.S. corporate tax rate |  |  |  |  |  |  | (7) | 52 | 23 |  |
Income tax expense (benefit) |  |  |  |  |  |  | 247 | 119 | 195 |  |
Decrease in net deferred tax assets resulting from the changes in the Tax Cuts and Jobs Act of 2017 |  |  |  |  |  |  | $ 83 | $ 0 | 0 |  |
Effective tax rate excluding the charge related to the exit of certain lines of business within subsidiaries and impact of the change in the U.S. corporate tax rate |  |  |  |  |  |  | 31.00% |  |  |  |
Effective tax rate excluding the charge related to the exit of certain lines of business within subsidiaries, acquisition of the noncontrolling interest, and restructuring of subsidiaries |  |  |  |  |  |  |  | 35.00% |  |  |
Income (losses) subject to tax in foreign jurisdictions |  |  |  |  |  |  | $ (58) | $ (160) | (66) |  |
Operating loss carryforwards, valuation allowance |  |  |  | $ 27 |  |  | 27 |  |  |  |
Decrease in AFG's liability for uncertain tax positions, settlements |  |  |  |  |  |  | (1) | 0 | 0 |  |
Change in AFG's liability for uncertain tax positions of prior year |  |  |  |  |  |  | 0 | 0 | 0 |  |
Cash payments for income taxes |  |  |  |  |  |  | $ 194 | 308 | 234 |  |
Limitation on the use of tax credit carryforwards |  |  |  |  |  |  | A SRLY NOL can be used only by the entity that created it and only in years that both it and the consolidated group have taxable income. Approximately $13 million of AFG’s SRLY NOLs expired unutilized at December 31, 2017. |  |  |  |
AFG |  |  |  |  |  |  |  |  |  |  |
Income Tax Reconciliation [Line Items] |  |  |  |  |  |  |  |  |  |  |
Income tax expense (benefit) |  |  |  | (56) | $ (177) |  | $ 247 | 119 | 195 |  |
Amount of expense for penalties related to a tax position |  |  |  |  |  |  |  |  | 0 |  |
National Interstate |  |  |  |  |  |  |  |  |  |  |
Income Tax Reconciliation [Line Items] |  |  |  |  |  |  |  |  |  |  |
Income tax expense (benefit) |  |  |  |  | (66) |  |  |  |  |  |
Neon Capital Limited |  |  |  |  |  |  |  |  |  |  |
Income Tax Reconciliation [Line Items] |  |  |  |  |  |  |  |  |  |  |
Income tax expense (benefit) |  |  |  | (56) | (111) |  |  | (48) |  |  |
Increase (Decrease) in current taxes payable |  |  |  | (20) | (29) |  |  |  |  |  |
Charge for valuation allowance |  |  |  |  |  |  |  |  |  |  |
Income Tax Reconciliation [Line Items] |  |  |  |  |  |  |  |  |  |  |
Charge for valuation allowance against deferred tax assets |  |  |  |  |  |  | 80 |  |  |  |
Maximum | AFG |  |  |  |  |  |  |  |  |  |  |
Income Tax Reconciliation [Line Items] |  |  |  |  |  |  |  |  |  |  |
Unrecognized tax benefits that would impact the effective tax rate |  |  |  | 1 | 1 |  | 1 | 1 |  |  |
Interest included in tax provision |  |  |  |  |  |  | (1) | 1 | 1 |  |
Decrease in related accrued interest (are less than in 2017 and 2016) |  |  |  | 1 | 1 |  | 1 | 1 |  |  |
Amount of expense for penalties related to a tax position |  |  |  |  |  |  | 1 | 1 |  |  |
Income tax penalties accrued |  |  |  | 1 | $ 1 |  | 1 | 1 |  |  |
Amount Unutilized [Member] |  |  |  |  |  |  |  |  |  |  |
Income Tax Reconciliation [Line Items] |  |  |  |  |  |  |  |  |  |  |
Operating loss carryforwards |  |  |  | 13 |  |  | 13 |  |  |  |
Tax Cuts and Jobs Act of 2017 |  |  |  |  |  |  |  |  |  |  |
Income Tax Reconciliation [Line Items] |  |  |  |  |  |  |  |  |  |  |
Decrease in net deferred tax assets resulting from the changes in the Tax Cuts and Jobs Act of 2017 |  |  |  |  |  |  | 83 |  |  |  |
Tax Cuts and Jobs Act of 2017 | Neon Capital Limited |  |  |  |  |  |  |  |  |  |  |
Income Tax Reconciliation [Line Items] |  |  |  |  |  |  |  |  |  |  |
Decrease in the valuation allowance related to the change in the U.S. corporate tax rate |  |  |  |  |  |  | (61) |  |  |  |
United Kingdom |  |  |  |  |  |  |  |  |  |  |
Income Tax Reconciliation [Line Items] |  |  |  |  |  |  |  |  |  |  |
Operating loss carryforwards | [1] |  |  | $ 252 |  |  | 252 |  |  |  | £ 186
Property and casualty insurance |  |  |  |  |  |  |  |  |  |  |
Income Tax Reconciliation [Line Items] |  |  |  |  |  |  |  |  |  |  |
Change in AFG’s liability for uncertain tax positions due to the uncertainty in state taxation |  |  |  |  |  |  |  |  | $ 1 |  |
Tax payments due to resolved uncertainty in state taxation |  |  |  |  |  |  | 1 |  |  |  |
Property and casualty insurance | Other lines |  |  |  |  |  |  |  |  |  |  |
Income Tax Reconciliation [Line Items] |  |  |  |  |  |  |  |  |  |  |
Special charges related to the exit of certain lines of business within subsidiaries |  |  |  |  |  | $ 65 |  | 65 |  |  |
Property and casualty insurance | Other lines | Neon Capital Limited |  |  |  |  |  |  |  |  |  |  |
Income Tax Reconciliation [Line Items] |  |  |  |  |  |  |  |  |  |  |
Special charges related to the exit of certain lines of business within subsidiaries |  |  |  |  |  |  |  | $ 65 |  |  |
Neon Capital Limited |  |  |  |  |  |  |  |  |  |  |
Income Tax Reconciliation [Line Items] |  |  |  |  |  |  |  |  |  |  |
Payments to acquire controlling interest in subsidiaries |  |  | $ 75 |  |  |  |  |  |  |  |
Percentage ownership of subsidiary |  |  | 67.00% |  |  |  |  |  |  |  |
Payments to acquire additional interest in subsidiaries |  |  |  |  |  |  |  |  |  | $ 17 |
Subsequent Event | Tax Cuts and Jobs Act of 2017 |  |  |  |  |  |  |  |  |  |  |
Income Tax Reconciliation [Line Items] |  |  |  |  |  |  |  |  |  |  |
Statutory rate of income taxes |  | 21.00% |  |  |  |  |  |  |  |  |
Foreign underwriting losses |  |  |  |  |  |  |  |  |  |  |
Income Tax Reconciliation [Line Items] |  |  |  |  |  |  |  |  |  |  |
impact of foreign operations included in the provision for income taxes |  |  |  |  |  |  | $ 14 |  |  |  |
[1] | £186 million

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