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Acquisitions and Sale of Businesses - Narrative on acquisitions of subsidiaries (Details) - USD ($) $ / shares in Units, $ in Millions |  | 3 Months Ended | 12 Months Ended
--- | --- | --- | ---
Nov. 10, 2016 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Business Acquisitions [Line Items] |  |  |  |  |  |
Payments to acquire additional interest in subsidiaries |  |  |  | $ 0 | $ 315 | $ 0
Income tax expense (benefit) |  |  |  | 247 | 119 | 195
National Interstate |  |  |  |  |  |
Business Acquisitions [Line Items] |  |  |  |  |  |
Percentage ownership of National Interstate by noncontrolling owners prior to acquisition | 49.00% |  |  |  |  |
Payments to acquire additional interest in subsidiaries | $ 315 |  |  |  |  |
Acquisition price per share (USD per share) | $ 32.00 |  |  |  |  |
Cash dividend paid (USD per share) | $ 0.50 |  |  |  |  |
Payment of special cash dividends | $ 5 |  |  |  |  |
Payments for acquisition related costs |  |  | $ 10 |  |  |
AFG |  |  |  |  |  |
Business Acquisitions [Line Items] |  |  |  |  |  |
Income tax expense (benefit) |  | $ (56) | (177) | $ 247 | $ 119 | $ 195
AFG | National Interstate |  |  |  |  |  |
Business Acquisitions [Line Items] |  |  |  |  |  |
Reduction to capital surplus | $ (137) |  |  |  |  |
Income tax expense (benefit) |  |  | $ (66) |  |  |

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- Definition This element represents acquisition-related costs incurred to effect a business combination which costs have been expensed during the period. Such costs include finder's fees; advisory, legal, accounting, valuation, and other professional or consulting fees; general administrative costs, including the costs of maintaining an internal acquisitions department; and may include costs of registering and issuing debt and equity securities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 10 -Section 25 -Paragraph 23 -URI http://asc.fasb.org/extlink&oid=84229725&loc=d3e1043-128460 + Details
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