Net Income Per Share: (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Earnings Per Share [Abstract]
Net income	$ 6,428	$ 11,034	$ 18,963	$ 27,086
Weighted average number of common shares outstanding (for basic calculation)	15,319,000	14,684,000	15,220,000	14,603,000
Dilutive securities	169,000	640,000	277,000	536,000
Weighted average common and common equivalent shares outstanding (for diluted calculation)	15,488,000	15,324,000	15,497,000	15,139,000
EPS - basic	$ 0.42	$ 0.75	$ 1.25	$ 1.85
EPS - diluted	$ 0.42	$ 0.72	$ 1.22	$ 1.79
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount	211,000	0	235,000	8,000