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Acquisition of Business - Narrative (Details) - USD ($)
$ in Millions
Sep. 30, 2019
Jun. 30, 2019
Dec. 31, 2018
Jul. 02, 2018
Business Acquisition [Line Items]        
Goodwill $ 1,114.0   $ 1,112.4  
Infinity Property and Casualty Corporation        
Business Acquisition [Line Items]        
Business acquisition, percentage of voting interests acquired       100.00%
Business combination, recognized identifiable assets acquired and liabilities assumed, legal accruals, other liabilities and tax related balance   $ 2.7    
Business combination, recognized identifiable assets acquired and liabilities assumed, deferred tax liabilities, current   0.2    
Goodwill   $ 2.5   $ 791.0