Employee Benefit Plans - Reconciliation of Pension Assets (Details) - Defined Benefit Pension Plans - USD ($) $ in Millions	12 Months Ended
	May 31, 2017	May 31, 2016
Employee benefit plans
Fair value of plan assets	$ 111.0	$ 115.8
Change in the fair value of plan assets:
Fair value of plan assets at beginning of year	111.0	115.8
Fair value of plan assets at end of year	118.8	111.0
Total
Employee benefit plans
Fair value of plan assets	111.0	111.0
Change in the fair value of plan assets:
Fair value of plan assets at beginning of year	111.0
Fair value of plan assets at end of year	118.8	111.0
U.S. common stock | Total
Employee benefit plans
Fair value of plan assets	2.3	2.3
Change in the fair value of plan assets:
Fair value of plan assets at beginning of year	2.3
Fair value of plan assets at end of year		2.3
U.S. mutual funds | Total
Employee benefit plans
Fair value of plan assets	24.6	24.6
Change in the fair value of plan assets:
Fair value of plan assets at beginning of year	24.6
Fair value of plan assets at end of year	30.6	24.6
International common stock | Total
Employee benefit plans
Fair value of plan assets	0.1	0.1
Change in the fair value of plan assets:
Fair value of plan assets at beginning of year	0.1
Fair value of plan assets at end of year		0.1
International mutual funds | Total
Employee benefit plans
Fair value of plan assets	7.9	7.9
Change in the fair value of plan assets:
Fair value of plan assets at beginning of year	7.9
Fair value of plan assets at end of year	9.5	7.9
Government securities and corporate bond mutual funds | Total
Employee benefit plans
Fair value of plan assets	9.9	9.9
Change in the fair value of plan assets:
Fair value of plan assets at beginning of year	9.9
Fair value of plan assets at end of year	10.1	9.9
Funds-of-funds | Total
Employee benefit plans
Fair value of plan assets	57.7	57.7
Change in the fair value of plan assets:
Fair value of plan assets at beginning of year	57.7
Fair value of plan assets at end of year	58.9	57.7
Hedge funds | Total
Employee benefit plans
Fair value of plan assets	6.6	6.6
Change in the fair value of plan assets:
Fair value of plan assets at beginning of year	6.6
Fair value of plan assets at end of year	6.8	6.6
Cash and cash equivalents | Total
Employee benefit plans
Fair value of plan assets	1.9	1.9
Change in the fair value of plan assets:
Fair value of plan assets at beginning of year	1.9
Fair value of plan assets at end of year	2.9	1.9
Quoted prices in active markets (Level 1)
Employee benefit plans
Fair value of plan assets	46.7	46.7
Change in the fair value of plan assets:
Fair value of plan assets at beginning of year	46.7
Fair value of plan assets at end of year	53.1	46.7
Quoted prices in active markets (Level 1) | U.S. common stock
Employee benefit plans
Fair value of plan assets	2.3	2.3
Change in the fair value of plan assets:
Fair value of plan assets at beginning of year	2.3
Fair value of plan assets at end of year		2.3
Quoted prices in active markets (Level 1) | U.S. mutual funds
Employee benefit plans
Fair value of plan assets	24.6	24.6
Change in the fair value of plan assets:
Fair value of plan assets at beginning of year	24.6
Fair value of plan assets at end of year	30.6	24.6
Quoted prices in active markets (Level 1) | International common stock
Employee benefit plans
Fair value of plan assets	0.1	0.1
Change in the fair value of plan assets:
Fair value of plan assets at beginning of year	0.1
Fair value of plan assets at end of year		0.1
Quoted prices in active markets (Level 1) | International mutual funds
Employee benefit plans
Fair value of plan assets	7.9	7.9
Change in the fair value of plan assets:
Fair value of plan assets at beginning of year	7.9
Fair value of plan assets at end of year	9.5	7.9
Quoted prices in active markets (Level 1) | Government securities and corporate bond mutual funds
Employee benefit plans
Fair value of plan assets	9.9	9.9
Change in the fair value of plan assets:
Fair value of plan assets at beginning of year	9.9
Fair value of plan assets at end of year	10.1	9.9
Quoted prices in active markets (Level 1) | Cash and cash equivalents
Employee benefit plans
Fair value of plan assets	1.9	1.9
Change in the fair value of plan assets:
Fair value of plan assets at beginning of year	1.9
Fair value of plan assets at end of year	2.9	1.9
Significant other observable inputs (Level 2)
Employee benefit plans
Fair value of plan assets	50.2	50.2
Change in the fair value of plan assets:
Fair value of plan assets at beginning of year	50.2
Fair value of plan assets at end of year	51.6	50.2
Significant other observable inputs (Level 2) | Funds-of-funds
Employee benefit plans
Fair value of plan assets	50.2	50.2
Change in the fair value of plan assets:
Fair value of plan assets at beginning of year	50.2
Fair value of plan assets at end of year	51.6	50.2
Significant other unobservable inputs (Level 3)
Employee benefit plans
Fair value of plan assets	14.1	15.1
Change in the fair value of plan assets:
Fair value of plan assets at beginning of year	14.1	15.1
Sales	(0.8)
Return on plan assets related to assets still held	0.8	(1.0)
Fair value of plan assets at end of year	14.1	14.1
Significant other unobservable inputs (Level 3) | Funds-of-funds
Employee benefit plans
Fair value of plan assets	7.5	8.0
Change in the fair value of plan assets:
Fair value of plan assets at beginning of year	7.5	8.0
Sales	(0.8)
Return on plan assets related to assets still held	0.6	(0.5)
Fair value of plan assets at end of year	7.3	7.5
Significant other unobservable inputs (Level 3) | Hedge funds
Employee benefit plans
Fair value of plan assets	6.6	7.1
Change in the fair value of plan assets:
Fair value of plan assets at beginning of year	6.6	7.1
Return on plan assets related to assets still held	0.2	(0.5)
Fair value of plan assets at end of year	$ 6.8	$ 6.6