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Acquisition and Sale of Businesses - Narrative on Acquisition of Subsidiary (Details) - USD ($) $ / shares in Units, $ in Millions |  | 3 Months Ended | 9 Months Ended
--- | --- | --- | ---
Nov. 10, 2016 | Sep. 30, 2016 | Sep. 30, 2015 | Sep. 30, 2016 | Sep. 30, 2015
Business Acquisition |  |  |  |  |
Income tax expense (benefit) |  | $ 65 | $ 33 | $ 190 | $ 115
AFG |  |  |  |  |
Business Acquisition |  |  |  |  |
Percentage ownership of National Interstate by parent company |  | 51.00% |  | 51.00% |
Alan Spachman |  |  |  |  |
Business Acquisition |  |  |  |  |
Percentage ownership of National Interstate by noncontrolling owner |  | 10.00% |  | 10.00% |
Alan Spachman | Percentage represented based on the shares not owned by parent company |  |  |  |  |
Business Acquisition |  |  |  |  |
Percentage ownership of National Interstate by noncontrolling owner |  | 20.00% |  | 20.00% |
Scenario, Forecast | AFG |  |  |  |  |
Business Acquisition |  |  |  |  |
Reduction to capital surplus | $ (140) |  |  |  |
Income tax expense (benefit) | $ (66) |  |  |  |
Scenario, Forecast | National Interstate |  |  |  |  |
Business Acquisition |  |  |  |  |
Payments to acquire business (USD per share) | $ 32.00 |  |  |  |
Cash dividend paid (USD per share) | $ 0.50 |  |  |  |
Payments to acquire business | $ 320 |  |  |  |

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