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Stockholders' Equity (Details) - USD ($)
$ in Thousands
12 Months Ended
Apr. 27, 2016
Jan. 26, 2016
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Jan. 01, 2018
Jan. 01, 2016
Conversion of Stock [Line Items]              
Additional cash issued to class B common stock holders     $ 0 $ 0 $ 12,000    
Unrecognized excess tax benefits from additional paid-in capital to retained earnings           $ 0 $ 0
Number of treasury shares held     352,161 352,161      
Net unrealized investment gains     $ (2,162) $ 0 0    
Subsidiaries              
Conversion of Stock [Line Items]              
Number of treasury shares held     145,820 145,820      
Retained Earnings              
Conversion of Stock [Line Items]              
Unrecognized excess tax benefits from additional paid-in capital to retained earnings           $ 3,592 $ 631
Retained Earnings | ASU 2016-01              
Conversion of Stock [Line Items]              
Net unrealized investment gains     $ 4,600        
Conversion, Class B Common Stock to Common Stock              
Conversion of Stock [Line Items]              
Additional cash issued to class B common stock holders $ 12,000 $ 12,000 $ 12,000   $ 12,000    
Conversion, Class B Common Stock to Common Stock | Class B Common Stock              
Conversion of Stock [Line Items]              
Number of shares converted 1,050,012 1,050,012          
Conversion, Class B Common Stock to Common Stock | Common Stock              
Conversion of Stock [Line Items]              
Number of shares issued 1,050,012 1,050,012