Annual report pursuant to Section 13 and 15(d)

Shareholders' Equity, Stock Options and Net Income per Share - Common and Preferred Stock (Details)

v3.8.0.1
Shareholders' Equity, Stock Options and Net Income per Share - Common and Preferred Stock (Details) - USD ($)
$ / shares in Units, $ in Thousands
 3 Months Ended 12 Months Ended
Jul. 21, 2016
Mar. 31, 2018
Dec. 31, 2017
Sep. 30, 2017
Jun. 30, 2017
Mar. 31, 2017
Dec. 31, 2016
Sep. 30, 2016
Jun. 30, 2016
Mar. 31, 2016
Dec. 31, 2015
Sep. 30, 2015
Jun. 30, 2015
Mar. 31, 2015
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
May 31, 2016
Dec. 31, 2008
May 31, 2008
Preferred Stock [Abstract]                                        
Preferred stock, shares authorized     5,000,000       5,000,000               5,000,000 5,000,000        
Preferred stock par value per share     $ 0.01       $ 0.01               $ 0.01 $ 0.01        
Cash Dividends [Abstract]                                        
Quarterly cash dividend per share, common stock   $ 0.15 $ 0.15 $ 0.15 $ 0.15 $ 0.15 $ 0.15 $ 0.12 $ 0.12 $ 0.12 $ 0.12 $ 0.12 $ 0.12 $ 0.12            
Repurchase of Common Stock [Abstract]                                        
Repurchase of common stock (repurchase program)                             $ 48,983 $ 39,983 $ 19,992      
Common Stock [Member]                                        
Share-based Compensation [Abstract]                                        
Number of shares authorized under the plan     7,500,000                       7,500,000       4,500,000  
Additional authorized shares for employee stock awards     1,691,567                       1,691,567     2,000,000   3,000,000
Minimum [Member] | Employee Stock Option [Member]                                        
Share-based Compensation [Abstract]                                        
Expected life of options (in years)                             7 years          
Option award vesting period                             1 year          
Maximum [Member] | Employee Stock Option [Member]                                        
Share-based Compensation [Abstract]                                        
Expected life of options (in years)                             10 years          
Option award vesting period                             5 years          
2016 Stock Repurchase Plan [Member] | Common Stock [Member]                                        
Repurchase of Common Stock [Abstract]                                        
Share repurchase program, authorized shares 3,000,000                                      
Stock Repurchased During Period, Shares                             947,819          
Repurchase of common stock (repurchase program)                             $ 48,983          
Treasury stock acquired, average cost per share                             $ 51.68          
Remaining shares authorized under share repurchase program                             1,818,665