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Acquisition of Business - Narrative on Acquisition of Subsidiary (Details) - National Interstate
$ / shares in Units, $ in Millions
Nov. 10, 2016
USD ($)
$ / shares
Business Acquisition  
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