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Stockholders' Equity (Schedule of Stock Options) (Details) - $ / shares
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Number of Option Shares      
Exercised (in shares) (239,854) (395,274) (505,312)
Equity Plan      
Number of Option Shares      
Balance at beginning of the period (in shares) 26,880 212,030 453,331
Exercised (in shares) (26,180) (183,600) (214,601)
Forfeited (in shares) (700) (1,550) (26,700)
Reduction for cash dividend (in shares)     0
Balance at end of the period (in shares) 0 26,880 212,030
Option exercisable (in shares) 0    
Weighted Average Exercise Price      
Balance at beginning of the period (in usd per share) $ 17.17 $ 15.38 $ 16.89
Exercised (in usd per share) 17.17 15.10 14.48
Forfeited (in usd per share) 17.17 17.17 14.30
Reduction for cash dividend (in usd per share)     (2.00)
Balance at end of the period (in usd per share) 0.00 $ 17.17 $ 15.38
Options exercisable (in usd per share) $ 0.00