Net Income Per Share: (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Mar. 31, 2014
Earnings Per Share [Abstract]
Net income	$ 9,686us-gaap_ProfitLoss	$ 6,657us-gaap_ProfitLoss
Weighted average number of common shares outstanding (for basic calculation)	15,693,000	15,097,000
Dilutive securities	541,000	298,000
Weighted average common and common equivalent shares outstanding (for diluted calculation)	16,234,000	15,395,000
EPS - basic	$ 0.62us-gaap_EarningsPerShareBasic	$ 0.44us-gaap_EarningsPerShareBasic
EPS - diluted	$ 0.60us-gaap_EarningsPerShareDiluted	$ 0.43us-gaap_EarningsPerShareDiluted
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount	76,000	237,000

Securities (including those issuable pursuant to contingent stock agreements) that could potentially dilute basic earnings per share (EPS) or earnings per unit (EPU) in the future that were not included in the computation of diluted EPS or EPU because to do so would increase EPS or EPU amounts or decrease loss per share or unit amounts for the period presented.

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The amount of net income (loss) for the period per each share of common stock or unit outstanding during the reporting period.

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The amount of net income (loss) for the period available to each share of common stock or common unit outstanding during the reporting period and to each share or unit that would have been outstanding assuming the issuance of common shares or units for all dilutive potential common shares or units outstanding during the reporting period.

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Additional shares included in the calculation of diluted EPS as a result of the potentially dilutive effect of share based payment arrangements using the treasury stock method.

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The consolidated profit or loss for the period, net of income taxes, including the portion attributable to the noncontrolling interest.

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Number of [basic] shares or units, after adjustment for contingently issuable shares or units and other shares or units not deemed outstanding, determined by relating the portion of time within a reporting period that common shares or units have been outstanding to the total time in that period.

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