Income Taxes (Components of Deferred Tax Assets and Liabilities) (Details) - USD ($) $ in Millions	May 31, 2016	May 31, 2015
Income Taxes [Abstract]
Compensation and employee benefit liabilities	$ 21.4	$ 20.9
Other current liabilities	7.9	6.9
Tax credit carry forward	40.0	40.1
Depreciation	8.8	11.9
Stock-based compensation	22.0	21.6
Tax benefit of uncertain tax positions	18.6	15.7
Other	7.1	9.2
Gross deferred tax assets	125.8	126.3
Capitalized software	56.3	53.6
Depreciation	2.5	3.8
Goodwill and intangible assets	58.5	52.0
Revenue not subject to current taxes	13.3	13.1
Unrealized gains on available-for-sale securities	17.3	5.3
Other		0.3
Gross deferred tax liabilities	147.9	128.1
Net deferred tax liability	$ (22.1)	$ (1.8)