Employee Benefit Plans - Reconciliation of Pension Assets (Details) - Defined Benefit Pension Plans - USD ($) $ in Millions	12 Months Ended
	May 31, 2016	May 31, 2015
Employee benefit plans
Fair value of plan assets	$ 115.8	$ 116.0
Reconciliation of pension assets measured at fair value
Fair value of plan assets at beginning of year	115.8	116.0
Fair value of plan assets at end of year	111.0	115.8
Total
Employee benefit plans
Fair value of plan assets	115.8	115.8
Reconciliation of pension assets measured at fair value
Fair value of plan assets at beginning of year	115.8
Fair value of plan assets at end of year	111.0	115.8
U.S. common stock | Total
Employee benefit plans
Fair value of plan assets	3.3	3.3
Reconciliation of pension assets measured at fair value
Fair value of plan assets at beginning of year	3.3
Fair value of plan assets at end of year	2.3	3.3
U.S. mutual funds | Total
Employee benefit plans
Fair value of plan assets	26.8	26.8
Reconciliation of pension assets measured at fair value
Fair value of plan assets at beginning of year	26.8
Fair value of plan assets at end of year	24.6	26.8
International common stock | Total
Employee benefit plans
Fair value of plan assets	0.1	0.1
Reconciliation of pension assets measured at fair value
Fair value of plan assets at beginning of year	0.1
Fair value of plan assets at end of year	0.1	0.1
International mutual funds | Total
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
Fair value of plan assets at end of year	7.9	9.1
Government securities and corporate bond mutual funds | Total
Employee benefit plans
Fair value of plan assets	10.3	10.3
Reconciliation of pension assets measured at fair value
Fair value of plan assets at beginning of year	10.3
Fair value of plan assets at end of year	9.9	10.3
Funds-of-funds | Total
Employee benefit plans
Fair value of plan assets	57.1	57.1
Reconciliation of pension assets measured at fair value
Fair value of plan assets at beginning of year	57.1
Fair value of plan assets at end of year	57.7	57.1
Hedge funds | Total
Employee benefit plans
Fair value of plan assets	7.1	7.1
Reconciliation of pension assets measured at fair value
Fair value of plan assets at beginning of year	7.1
Fair value of plan assets at end of year	6.6	7.1
Cash and cash equivalents | Total
Employee benefit plans
Fair value of plan assets	2.0	2.0
Reconciliation of pension assets measured at fair value
Fair value of plan assets at beginning of year	2.0
Fair value of plan assets at end of year	1.9	2.0
Quoted prices in active markets (Level 1)
Employee benefit plans
Fair value of plan assets	51.6	51.6
Reconciliation of pension assets measured at fair value
Fair value of plan assets at beginning of year	51.6
Fair value of plan assets at end of year	46.7	51.6
Quoted prices in active markets (Level 1) | U.S. common stock
Employee benefit plans
Fair value of plan assets	3.3	3.3
Reconciliation of pension assets measured at fair value
Fair value of plan assets at beginning of year	3.3
Fair value of plan assets at end of year	2.3	3.3
Quoted prices in active markets (Level 1) | U.S. mutual funds
Employee benefit plans
Fair value of plan assets	26.8	26.8
Reconciliation of pension assets measured at fair value
Fair value of plan assets at beginning of year	26.8
Fair value of plan assets at end of year	24.6	26.8
Quoted prices in active markets (Level 1) | International common stock
Employee benefit plans
Fair value of plan assets	0.1	0.1
Reconciliation of pension assets measured at fair value
Fair value of plan assets at beginning of year	0.1
Fair value of plan assets at end of year	0.1	0.1
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
Fair value of plan assets at end of year	7.9	9.1
Quoted prices in active markets (Level 1) | Government securities and corporate bond mutual funds
Employee benefit plans
Fair value of plan assets	10.3	10.3
Reconciliation of pension assets measured at fair value
Fair value of plan assets at beginning of year	10.3
Fair value of plan assets at end of year	9.9	10.3
Quoted prices in active markets (Level 1) | Cash and cash equivalents
Employee benefit plans
Fair value of plan assets	2.0	2.0
Reconciliation of pension assets measured at fair value
Fair value of plan assets at beginning of year	2.0
Fair value of plan assets at end of year	1.9	2.0
Significant other observable inputs (Level 2)
Employee benefit plans
Fair value of plan assets	49.1	49.1
Reconciliation of pension assets measured at fair value
Fair value of plan assets at beginning of year	49.1
Fair value of plan assets at end of year	50.2	49.1
Significant other observable inputs (Level 2) | Funds-of-funds
Employee benefit plans
Fair value of plan assets	49.1	49.1
Reconciliation of pension assets measured at fair value
Fair value of plan assets at beginning of year	49.1
Fair value of plan assets at end of year	50.2	49.1
Significant other unobservable inputs (Level 3)
Employee benefit plans
Fair value of plan assets	15.1	11.1
Reconciliation of pension assets measured at fair value
Fair value of plan assets at beginning of year	15.1	11.1
Purchases		3.2
Return on plan assets related to assets still held	(1.0)	0.8
Fair value of plan assets at end of year	14.1	15.1
Significant other unobservable inputs (Level 3) | Funds-of-funds
Employee benefit plans
Fair value of plan assets	8.0	7.5
Reconciliation of pension assets measured at fair value
Fair value of plan assets at beginning of year	8.0	7.5
Return on plan assets related to assets still held	(0.5)	0.5
Fair value of plan assets at end of year	7.5	8.0
Significant other unobservable inputs (Level 3) | Hedge funds
Employee benefit plans
Fair value of plan assets	7.1	3.6
Reconciliation of pension assets measured at fair value
Fair value of plan assets at beginning of year	7.1	3.6
Purchases		3.2
Return on plan assets related to assets still held	(0.5)	0.3
Fair value of plan assets at end of year	$ 6.6	$ 7.1