Fair Value Measurements (Details) (Fair Value, Measurements, Recurring [Member], USD $) In Thousands, unless otherwise specified	Aug. 31, 2012	May 31, 2012
Investments
Total assets at fair value	$ 12,517	$ 16,505
CD's and money market accounts [Member]
Investments
Total assets at fair value	2,520	2,520
US treasury bills and notes [Member]
Investments
Total assets at fair value	9,997	14,255
Quoted Prices in Active Markets (Level 1) [Member]
Investments
Total assets at fair value	11,353	16,086
Quoted Prices in Active Markets (Level 1) [Member] | CD's and money market accounts [Member]
Investments
Total assets at fair value	1,356	1,831
Quoted Prices in Active Markets (Level 1) [Member] | US treasury bills and notes [Member]
Investments
Total assets at fair value	9,997	14,255
Other Observable Inputs (Level 2) [Member]
Investments
Total assets at fair value	1,164	419
Other Observable Inputs (Level 2) [Member] | CD's and money market accounts [Member]
Investments
Total assets at fair value	1,164	419
Unobservable Inputs (Level 3) [Member]
Investments
Total assets at fair value
Unobservable Inputs (Level 3) [Member] | CD's and money market accounts [Member]
Investments
Total assets at fair value
Unobservable Inputs (Level 3) [Member] | US treasury bills and notes [Member]
Investments
Total assets at fair value