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Stockholders' Equity:Stock Repurchase (Details) - USD ($)
$ / shares in Units, $ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Feb. 25, 2015
Equity, Class of Treasury Stock [Line Items]        
Treasury Stock, Value, Acquired, Cost Method   $ 10,268 $ 23,711  
ShareRepurchaseProgram2015 [Member]        
Equity, Class of Treasury Stock [Line Items]        
Stock Repurchase Program, Number of Shares Authorized to be Repurchased       2,000,000
Treasury stock, shares acquired 0 580,100 722,911  
Treasury Stock, Value, Acquired, Cost Method   $ 10,300 $ 23,700  
Treasury Stock Acquired, Average Cost Per Share   $ 17.70 $ 32.80  
Stock Repurchase Program, Remaining Number of Shares Authorized to be Repurchased 696,989