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Employee Benefit Plans: (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Feb. 28, 2013
Feb. 01, 1999
401(k) Plan          
Employee Benefit Plans:          
Number of common stock shares reserved for issuance (in shares)         150,000
Number of additional common stock shares reserved for issuance (in shares)       500,000  
Employer match of employee contributions of first 3% of eligible compensation (as a percent) 100.00%        
Percentage of eligible compensation, matched 100% by employer (as a percent) 3.00%        
Employer match of employee contributions of next 2% of eligible compensation (as a percent) 50.00%        
Percentage of eligible compensation, matched 50% by employer (as a percent) 2.00%        
Employer contribution $ 3,299 $ 3,253 $ 3,017    
Deferred Compensation Plans          
Employee Benefit Plans:          
Employer contribution $ 933 $ 845 $ 843