Net Income Per Share: (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Earnings Per Share [Abstract]
Net income	$ 8,685	$ 7,601
Weighted average number of common shares outstanding (for basic calculation)	14,507,000	13,956,000
Dilutive securities	546,000	362,000
Weighted average common and common equivalent shares outstanding (for diluted calculation)	15,053,000	14,318,000
EPS - basic	$ 0.60	$ 0.54
EPS - diluted	$ 0.58	$ 0.53
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount	142,000	141,000