Investments Securities (Details Textual) (USD $) In Millions, unless otherwise specified	Feb. 28, 2014	May 31, 2013
Federal Agricultural Mortgage Corporation Class A common stock
Investment [Line Items]
Investment at fair value	$ 2.1	$ 2.2
Unrealized gain recorded in accumulated other comprehensive income, included in common stock carrying value	1.6	1.7
Federal Agricultural Mortgage Corporation Series A preferred stock
Investment [Line Items]
Investment at fair value	27.0	29.0
Cost of purchases, included in preferred stock carrying value	30
Cost of purchases, included in common stock carrying value	0.5
Unrealized loss recorded in accumulated other comprehensive income, included in preferred stock carrying value	$ 3	$ 1