Investments - Summary of Investment Income (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
May 31, 2013	May 31, 2012	May 31, 2011
Investments, Debt and Equity Securities [Abstract]
Interest income	$ 0.1	$ 0.4	$ 0.6
Dividend income	0.2	0.2	0.1
Net realized gains	0.2	2.0	2.6
Total investment income	$ 0.5	$ 2.6	$ 3.3

- Definition

This item represents investment income derived from investments in debt and equity securities consisting of interest income earned from investments in debt securities and on cash and cash equivalents, dividend income from investments in equity securities, and income or expense derived from the amortization of investment related discounts or premiums, respectively, net of related investment expenses. This item does not include realized or unrealized gains or losses on the sale or holding of investments in debt and equity securities required to be included in earnings for the period or for other than temporary losses related to investments in debt and equity securities which are included in realized losses in the period recognized, and does not include investment income from real or personal property, such as rental income.

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