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Business Combinations (Details)
$ in Millions
12 Months Ended
Jan. 01, 2014
USD ($)
Dec. 31, 2016
Business
Dec. 31, 2015
Business
Dec. 31, 2014
USD ($)
Business Combinations [Abstract]        
Number of entities acquired | Business   0 0  
Eastern Insurance Holdings        
Business Acquisition [Line Items]        
Voting interests acquired (percent) 100.00%      
Total purchase consideration $ 205.0      
Expenses related to the purchase of business       $ 2.2