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Subsequent Event (Details Textual) - USD ($)
$ / shares in Units, $ in Millions
3 Months Ended 6 Months Ended
Jul. 25, 2016
Jun. 30, 2016
Jun. 30, 2015
Jun. 30, 2016
Jun. 30, 2015
Subsequent event [Line Items]          
Income tax expense (benefit)   $ 73 $ 77 $ 125 $ 82
AFG [Member] | Subsequent event [Member]          
Subsequent event [Line Items]          
Percentage ownership of National Interstate by parent company 51.00%        
Income tax expense (benefit) $ (64)        
National Interstate [Member] | Subsequent event [Member]          
Subsequent event [Line Items]          
Payments to acquire business (USD per share) $ 32.00        
Cash dividend paid (USD per share) $ 0.50        
Payments to acquire business $ 320        
Alan Spachman [Member] | Subsequent event [Member]          
Subsequent event [Line Items]          
Percentage ownership of National Interstate by noncontrolling owner 10.00%        
Percentage represented based on the shares not owned by parent company [Member] | Alan Spachman [Member] | Subsequent event [Member]          
Subsequent event [Line Items]          
Percentage ownership of National Interstate by noncontrolling owner 20.00%