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Condensed Statements of Comprehensive Margin (USD $)
In Thousands, unless otherwise specified
3 Months Ended 6 Months Ended
Jun. 30, 2013
Jun. 30, 2012
Jun. 30, 2013
Jun. 30, 2012
Condensed Statements of Comprehensive Margin        
Net margin $ 24,502 $ 10,878 $ 46,526 $ 24,398
Other comprehensive margin:        
Unrealized (loss) gain on available-for-sale securities (1,090) 120 (1,302) 828
Total comprehensive margin $ 23,412 $ 10,998 $ 45,224 $ 25,226