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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