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Summary of Significant Accounting Policies - Summary of our comprehensive income (Details) (USD $)
In Thousands, unless otherwise specified
 3 Months Ended 12 Months Ended
Mar. 31, 2013
Dec. 31, 2012
Sep. 30, 2012
Jun. 30, 2012
Mar. 31, 2012
Dec. 31, 2011
Sep. 30, 2011
Jun. 30, 2011
Mar. 31, 2013
Mar. 31, 2012
Mar. 31, 2011
Accounting Policies [Abstract]                      
Net income $ 4,602 $ 4,577 $ 4,034 $ 3,952 $ 3,993 $ 3,898 $ 3,930 $ 4,837 $ 17,165 $ 16,659 $ 20,871
Net change in unrealized gain (loss) on short term investments                 (46) 75 (31)
Net change in unrealized gain (loss) and redemptions on long term investments                   110 (2)
Comprehensive income                 $ 17,119 $ 16,844 $ 20,838