Employee Benefit Plans (Details 2) - Defined Benefit Pension Plans - USD ($) $ in Millions	12 Months Ended
	May. 31, 2015	May. 31, 2014
Employee benefit plans
Fair value of plan assets	$ 116.0	$ 101.6
Reconciliation of pension assets measured at fair value
Fair value of plan assets at beginning of year	116.0	101.6
Return on plan assets related to:
Fair value of plan assets at end of year	115.8	116.0
Quoted prices in active markets (Level 1)
Employee benefit plans
Fair value of plan assets	54.3	54.3
Reconciliation of pension assets measured at fair value
Fair value of plan assets at beginning of year	54.3
Return on plan assets related to:
Fair value of plan assets at end of year	51.6	54.3
Quoted prices in active markets (Level 1) | U.S. common stock
Employee benefit plans
Fair value of plan assets	6.5	6.5
Reconciliation of pension assets measured at fair value
Fair value of plan assets at beginning of year	6.5
Return on plan assets related to:
Fair value of plan assets at end of year	3.3	6.5
Quoted prices in active markets (Level 1) | U.S. mutual funds
Employee benefit plans
Fair value of plan assets	24.0	24.0
Reconciliation of pension assets measured at fair value
Fair value of plan assets at beginning of year	24.0
Return on plan assets related to:
Fair value of plan assets at end of year	26.8	24.0
Quoted prices in active markets (Level 1) | International common stock
Employee benefit plans
Fair value of plan assets	0.6	0.6
Reconciliation of pension assets measured at fair value
Fair value of plan assets at beginning of year	0.6
Return on plan assets related to:
Fair value of plan assets at end of year	0.1	0.6
Quoted prices in active markets (Level 1) | International mutual funds
Employee benefit plans
Fair value of plan assets	9.1	9.1
Reconciliation of pension assets measured at fair value
Fair value of plan assets at beginning of year	9.1
Return on plan assets related to:
Fair value of plan assets at end of year	9.1	9.1
Quoted prices in active markets (Level 1) | Government securities and corporate bond mutual funds
Employee benefit plans
Fair value of plan assets	11.0	11.0
Reconciliation of pension assets measured at fair value
Fair value of plan assets at beginning of year	11.0
Return on plan assets related to:
Fair value of plan assets at end of year	10.3	11.0
Quoted prices in active markets (Level 1) | Cash and cash equivalents
Employee benefit plans
Fair value of plan assets	3.1	3.1
Reconciliation of pension assets measured at fair value
Fair value of plan assets at beginning of year	3.1
Return on plan assets related to:
Fair value of plan assets at end of year	2.0	3.1
Significant other observable inputs (Level 2)
Employee benefit plans
Fair value of plan assets	50.6	50.6
Reconciliation of pension assets measured at fair value
Fair value of plan assets at beginning of year	50.6
Return on plan assets related to:
Fair value of plan assets at end of year	49.1	50.6
Significant other observable inputs (Level 2) | Funds-of-funds
Employee benefit plans
Fair value of plan assets	50.6	50.6
Reconciliation of pension assets measured at fair value
Fair value of plan assets at beginning of year	50.6
Return on plan assets related to:
Fair value of plan assets at end of year	49.1	50.6
Significant other unobservable inputs (Level 3)
Employee benefit plans
Fair value of plan assets	11.1	10.1
Reconciliation of pension assets measured at fair value
Fair value of plan assets at beginning of year	11.1	10.1
Sales		(4.7)
Purchases	3.2	5.4
Return on plan assets related to:
Assets sold		(0.3)
Assets still held	0.8	0.6
Fair value of plan assets at end of year	15.1	11.1
Significant other unobservable inputs (Level 3) | Hedge funds
Employee benefit plans
Fair value of plan assets	3.6	2.4
Reconciliation of pension assets measured at fair value
Fair value of plan assets at beginning of year	3.6	2.4
Sales		(2.4)
Purchases	3.2	3.6
Return on plan assets related to:
Assets still held	0.3
Fair value of plan assets at end of year	7.1	3.6
Significant other unobservable inputs (Level 3) | Funds-of-funds
Employee benefit plans
Fair value of plan assets	7.5	7.7
Reconciliation of pension assets measured at fair value
Fair value of plan assets at beginning of year	7.5	7.7
Sales		(2.3)
Purchases		1.8
Return on plan assets related to:
Assets sold		(0.3)
Assets still held	0.5	0.6
Fair value of plan assets at end of year	8.0	7.5
Total
Employee benefit plans
Fair value of plan assets	116.0	116.0
Reconciliation of pension assets measured at fair value
Fair value of plan assets at beginning of year	116.0
Return on plan assets related to:
Fair value of plan assets at end of year	115.8	116.0
Total | U.S. common stock
Employee benefit plans
Fair value of plan assets	6.5	6.5
Reconciliation of pension assets measured at fair value
Fair value of plan assets at beginning of year	6.5
Return on plan assets related to:
Fair value of plan assets at end of year	3.3	6.5
Total | U.S. mutual funds
Employee benefit plans
Fair value of plan assets	24.0	24.0
Reconciliation of pension assets measured at fair value
Fair value of plan assets at beginning of year	24.0
Return on plan assets related to:
Fair value of plan assets at end of year	26.8	24.0
Total | International common stock
Employee benefit plans
Fair value of plan assets	0.6	0.6
Reconciliation of pension assets measured at fair value
Fair value of plan assets at beginning of year	0.6
Return on plan assets related to:
Fair value of plan assets at end of year	0.1	0.6
Total | International mutual funds
Employee benefit plans
Fair value of plan assets	9.1	9.1
Reconciliation of pension assets measured at fair value
Fair value of plan assets at beginning of year	9.1
Return on plan assets related to:
Fair value of plan assets at end of year	9.1	9.1
Total | Government securities and corporate bond mutual funds
Employee benefit plans
Fair value of plan assets	11.0	11.0
Reconciliation of pension assets measured at fair value
Fair value of plan assets at beginning of year	11.0
Return on plan assets related to:
Fair value of plan assets at end of year	10.3	11.0
Total | Hedge funds
Employee benefit plans
Fair value of plan assets	3.6	3.6
Reconciliation of pension assets measured at fair value
Fair value of plan assets at beginning of year	3.6
Return on plan assets related to:
Fair value of plan assets at end of year	7.1	3.6
Total | Funds-of-funds
Employee benefit plans
Fair value of plan assets	58.1	58.1
Reconciliation of pension assets measured at fair value
Fair value of plan assets at beginning of year	58.1
Return on plan assets related to:
Fair value of plan assets at end of year	57.1	58.1
Total | Cash and cash equivalents
Employee benefit plans
Fair value of plan assets	3.1	3.1
Reconciliation of pension assets measured at fair value
Fair value of plan assets at beginning of year	3.1
Return on plan assets related to:
Fair value of plan assets at end of year	$ 2.0	$ 3.1