Net Income Per Share: (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		6 Months Ended	9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Jun. 30, 2013	Sep. 30, 2013	Sep. 30, 2012
Earnings Per Share [Abstract]
Net income	$ 11,034	$ 12,188	$ 28,938	$ 27,086	$ 28,938
Weighted average number of common shares outstanding (for basic calculation)	14,684,000	14,321,000		14,603,000	14,153,000
Dilutive securities	640,000	505,000	460,000	536,000
Weighted average common and common equivalent shares outstanding (for diluted calculation)	15,324,000	14,826,000	14,613,000	15,139,000	14,613,000
EPS - basic	$ 0.75	$ 0.85	$ 2.04	$ 1.85	$ 2.04
EPS - diluted	$ 0.72	$ 0.82	$ 1.98	$ 1.79	$ 1.98
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount	0	0		8,000	5,000