Net Income Per Share: (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Earnings Per Share [Abstract]
Net income	$ 7,367	$ 9,149	$ 16,052	$ 16,750
Weighted average number of common shares outstanding (for basic calculation)	14,617,000	14,179,000	14,562,000	14,067,000
Dilutive securities	599,000	441,000	585,000	417,000
Weighted average common and common equivalent shares outstanding (for diluted calculation)	15,216,000	14,620,000	15,147,000	14,484,000
EPS - basic	$ 0.50	$ 0.65	$ 1.10	$ 1.19
EPS - diluted	$ 0.48	$ 0.63	$ 1.06	$ 1.16
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount		5,000		3,000