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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Earnings Per Share (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Common Class A [Member]
Numerator:
Allocation of net income									$ 18,455	$ 390	$ 9,183
Denominator:
Weighted-average shares outstanding									20,031,455	19,997,146	19,961,274
Basic earnings per share	$ 0.25	$ 0.27	$ 0.08	$ 0.32					$ 0.92	$ 0.02	$ 0.46
Basic and diluted earnings per share					$ (0.03)	$ 0.03	$ (0.07)	$ 0.09
Numerator:
Allocation of net income									18,455	390	9,183
Denominator:
Number of shares used in basic computation									20,031,455	19,997,146	19,961,274
Weighted-average effect of dilutive securities
Add: Director and employee stock options									274,103	36,499	17,794
Number of shares used in per share computations									20,305,558	20,033,645	19,979,068
Diluted earnings per share	$ 0.25	$ 0.27	$ 0.08	$ 0.31					$ 0.91	$ 0.02	$ 0.46
Common Class B [Member]
Numerator:
Allocation of net income									$ 4,638	$ 63	$ 2,284
Denominator:
Weighted-average shares outstanding									5,576,775	5,576,775	5,576,775
Basic earnings per share									$ 0.83	$ 0.01	$ 0.41
Basic and diluted earnings per share	$ 0.22	$ 0.25	$ 0.07	$ 0.29	$ (0.03)	$ 0.03	$ (0.06)	$ 0.08	$ 0.83	$ 0.01	$ 0.41
Denominator:
Number of shares used in basic computation									5,576,775	5,576,775	5,576,775
Weighted-average effect of dilutive securities
Diluted earnings per share									$ 0.83	$ 0.01	$ 0.41