Employee Benefit Plans - Reconciliation of Pension Assets (Details) - Defined Benefit Pension Plans - USD ($) $ in Millions	May 31, 2018	May 31, 2017	May 31, 2016
Employee benefit plans
Fair value of plan assets	$ 134.5	$ 118.8	$ 111.0
Total
Employee benefit plans
Fair value of plan assets	134.5	118.8
U.S. mutual funds | Total
Employee benefit plans
Fair value of plan assets	32.8	30.6
International mutual funds | Total
Employee benefit plans
Fair value of plan assets	10.4	9.5
Government securities and corporate bond mutual funds | Total
Employee benefit plans
Fair value of plan assets	15.7	10.1
Funds-of-funds | Total
Employee benefit plans
Fair value of plan assets	61.3	58.9
Hedge funds | Total
Employee benefit plans
Fair value of plan assets	4.4	6.8
Insurance annuities | Total
Employee benefit plans
Fair value of plan assets	7.8
Cash and cash equivalents | Total
Employee benefit plans
Fair value of plan assets	2.1	2.9
Quoted prices in active markets (Level 1)
Employee benefit plans
Fair value of plan assets	61.0	53.1
Quoted prices in active markets (Level 1) | U.S. mutual funds
Employee benefit plans
Fair value of plan assets	32.8	30.6
Quoted prices in active markets (Level 1) | International mutual funds
Employee benefit plans
Fair value of plan assets	10.4	9.5
Quoted prices in active markets (Level 1) | Government securities and corporate bond mutual funds
Employee benefit plans
Fair value of plan assets	15.7	10.1
Quoted prices in active markets (Level 1) | Cash and cash equivalents
Employee benefit plans
Fair value of plan assets	2.1	2.9
Significant other observable inputs (Level 2)
Employee benefit plans
Fair value of plan assets	53.6	51.6
Significant other observable inputs (Level 2) | Funds-of-funds
Employee benefit plans
Fair value of plan assets	53.6	51.6
Significant other unobservable inputs (Level 3)
Employee benefit plans
Fair value of plan assets	19.9	14.1	14.1
Significant other unobservable inputs (Level 3) | Funds-of-funds
Employee benefit plans
Fair value of plan assets	7.7	7.3	7.5
Significant other unobservable inputs (Level 3) | Hedge funds
Employee benefit plans
Fair value of plan assets	4.4	$ 6.8	$ 6.6
Significant other unobservable inputs (Level 3) | Insurance annuities
Employee benefit plans
Fair value of plan assets	$ 7.8