v3.8.0.1
Shareholders' Equity (Details) - USD ($)
$ in Thousands
 1 Months Ended 12 Months Ended
Dec. 31, 2013
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Feb. 13, 2018
Class of Stock [Line Items]          
Treasury Shares   20,733,010 19,303,578    
Additional authorized shares purchased   7,162,279      
Stock repurchased during period $ 306        
Repurchased shares, amount   $ (62,550) $ (34,525)    
Preferred stock, authorized (in shares)   1,000,000      
Preferred stock, issued (in shares)   0      
Treasury Stock          
Class of Stock [Line Items]          
Repurchased shares, amount   $ (62,550) (34,525)    
Additional Paid-in Capital          
Class of Stock [Line Items]          
Repurchased shares, amount   $ 0 $ 0    
Nonvoting Common Stock          
Class of Stock [Line Items]          
Common stock repurchased (in shares)   1,961,442 1,372,700 0  
Subsequent Event          
Class of Stock [Line Items]          
Authorized stock repurchase program, amount         $ 500,000