Net Income Per Share: (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Earnings Per Share [Abstract]
Net income	$ 12,188	$ 9,261	$ 28,938	$ 26,827
Weighted average number of common shares outstanding (for basic calculation)	14,321,000	13,932,000	14,153,000	13,826,000
Dilutive securities	505,000	252,000	460,000	343,000
Weighted average common and common equivalent shares outstanding (for diluted calculation)	14,826,000	14,184,000	14,613,000	14,169,000
EPS - basic	$ 0.85	$ 0.66	$ 2.04	$ 1.94
EPS - diluted	$ 0.82	$ 0.65	$ 1.98	$ 1.89
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount	0	295,000	5,000	189,000