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Significant Accounting Policies (Details) - USD ($)
$ in Thousands
 3 Months Ended
Mar. 31, 2016
Mar. 31, 2015
Schedule of activity in the Graduation Fund    
Balance at beginning of period $ 20,937 $ 9,706
Revenue deferred 5,564 3,603
Benefit redeemed (3,236) (634)
Balance at end of period $ 23,265 $ 12,675