Annual report pursuant to Section 13 and 15(d)

Shareholders' Equity (Repurchase Program and Warrants) (Details)

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Shareholders' Equity (Repurchase Program and Warrants) (Details) - USD ($)
$ in Thousands
 12 Months Ended 130 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2017
Nov. 30, 2016
Nov. 30, 2014
Oct. 31, 2011
Mar. 31, 2007
Class of Stock [Line Items]                
Repurchase and retirement of common stock $ 5,293 $ 14,749 $ 16,355          
Shares repurchased, value $ 850              
Warrants outstanding (in shares) 0     0        
Warrant exercise (in shares) 2,000,000 13,236,333 646,000          
Intrinsic value of warrant exercised $ 1,830 $ 9,320 $ 966          
Exercise of stock options, shares tendered in payment of exercise price (in shares) 19,658 846,789 196,518          
Common Stock                
Class of Stock [Line Items]                
Repurchase and retirement of common stock (in shares) 2,088,460 6,132,124 5,673,415          
Exercise of warrants (in shares) 2,000,000 12,389,544 449,482          
Repurchase Program                
Class of Stock [Line Items]                
Shares authorized for repurchase (in shares)         10,000,000 10,000,000    
Repurchase and retirement of common stock (in shares) 1,850,215 5,230,433   19,350,215        
Repurchase and retirement of common stock       $ 49,411        
Shares repurchased, value $ 4,721 $ 12,711            
Shares availabe for purchase under repurchase program (in shares) 8,149,785     8,149,785        
Repurchase Program | Common Stock                
Class of Stock [Line Items]                
Shares authorized for repurchase (in shares)               2,500,000
Shares authorized for additional repurchase (in shares)             5,000,000