Consolidated Statements Of Changes In Shareholders' Equity - USD ($)
Total
Common Stock
Additional Paid-in Capital
Retained Earnings
Accumulated Other Comprehensive Income, Net of Tax
Treasury Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]            
Cumulative effect of change in accounting principle $ (3,808,000) $ 0 $ 0 $ (3,808,000) $ 0 $ 0
Beginning Balance at Dec. 31, 2016 699,187,000 21,829,000 378,745,000 777,695,000 7,907,000 (486,990,000)
Increase (Decrease) in Stockholders' Equity [Roll Forward]            
Net earnings 10,510,000     10,510,000    
Net change in post-retirement benefit liability (8,000)       (8,000)  
Change in unrealized gain on investments 6,579,000       6,579,000  
Change in non-credit component of impairment losses on fixed maturities 22,000       22,000  
Comprehensive income 17,102,000          
Dividends paid to common shareholders (6,402,000)     (6,402,000)    
Shares issued and share-based compensation expense 1,449,000 18,000 1,430,000      
Acquisition of treasury stock (2,303,000)         (2,303,000)
Ending Balance at Mar. 31, 2017 705,225,000 21,848,000 380,175,000 777,994,000 14,500,000 (489,293,000)
Increase (Decrease) in Stockholders' Equity [Roll Forward]            
Net earnings 34,214,000     34,214,000    
Net change in post-retirement benefit liability (124,000)       (124,000)  
Change in unrealized gain on investments 5,074,000       5,074,000  
Change in non-credit component of impairment losses on fixed maturities 306,000       306,000  
Comprehensive income 39,469,000          
Dividends paid to common shareholders (19,131,000)     (19,131,000)    
Shares issued and share-based compensation expense 3,432,000 40,000 3,391,000      
Acquisition of treasury stock (13,182,000)         (13,182,000)
Ending Balance at Dec. 31, 2017 715,812,000 21,888,000 383,567,000 793,077,000 19,756,000 (502,475,000)
Increase (Decrease) in Stockholders' Equity [Roll Forward]            
Cumulative effect of change in accounting principle (10,000) 0 0 18,118,000 (18,128,000) 0
Net earnings 20,083,000     20,083,000    
Net change in post-retirement benefit liability (1,000)       (1,000)  
Change in unrealized gain on investments (12,980,000)       (12,980,000)  
Change in non-credit component of impairment losses on fixed maturities 152,000       152,000  
Comprehensive income 7,254,000          
Dividends paid to common shareholders (6,346,000)     (6,346,000)    
Shares issued and share-based compensation expense 1,800,000 21,000 1,779,000      
Acquisition of treasury stock (594,000)         (594,000)
Ending Balance at Mar. 31, 2018 $ 717,916,000 $ 21,908,000 $ 385,346,000 $ 824,932,000 $ (11,202,000) $ (503,069,000)