Accumulated Other Comprehensive Income - Reclassification out of AOCI (Details) - USD ($) $ in Millions	12 Months Ended
	May 31, 2020	May 31, 2019	May 31, 2018
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Settlement charge			$ (57.3)
Postretirement Benefits [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Settlement charge	$ 0.0	$ 0.0	0.0
Recognized net actuarial loss [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amounts reclassified from Accumulated other comprehensive income (loss)	0.8	0.7
Recognized net actuarial loss [Member] | Pension Plans [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amounts reclassified from Accumulated other comprehensive income (loss)	1.0	0.8
Recognized net actuarial loss [Member] | Postretirement Benefits [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amounts reclassified from Accumulated other comprehensive income (loss)	(0.2)	(0.1)
Amount reclassified from Accumulated other comprehensive income (loss) [Member] | Service cost [Member] | Pension Plans [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassifications, before tax	0.0	0.0	0.0
Amount reclassified from Accumulated other comprehensive income (loss) [Member] | Service cost [Member] | Postretirement Benefits [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassifications, before tax	0.0	0.0	(0.0)
Amount reclassified from Accumulated other comprehensive income (loss) [Member] | Net amortization and deferrals [Member] | Pension Plans [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassifications, before tax	0.0	0.0	0.0
Amount reclassified from Accumulated other comprehensive income (loss) [Member] | Net amortization and deferrals [Member] | Postretirement Benefits [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassifications, before tax	(0.2)	(0.1)	0.0
Amount reclassified from Accumulated other comprehensive income (loss) [Member] | Lump sum settlement charge [Member] | Pension Plans [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassifications, before tax	0.0	0.0	55.0
Amount reclassified from Accumulated other comprehensive income (loss) [Member] | Lump sum settlement charge [Member] | Postretirement Benefits [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassifications, before tax	0.0	0.0	0.0
Amount reclassified from Accumulated other comprehensive income (loss) [Member] | Recognized net actuarial loss [Member] | Pension Plans [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassifications, before tax	1.0	0.8	2.1
Amount reclassified from Accumulated other comprehensive income (loss) [Member] | Recognized net actuarial loss [Member] | Postretirement Benefits [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassifications, before tax	0.0	0.0	0.1
Amount reclassified from Accumulated other comprehensive income (loss) [Member] | Accumulated Defined Benefit Plans Adjustment [Member] | Pension Plans [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Tax (benefit) expense	0.0	0.0	(22.3)
Amounts reclassified from Accumulated other comprehensive income (loss)	1.0	0.8	34.8
Amount reclassified from Accumulated other comprehensive income (loss) [Member] | Accumulated Defined Benefit Plans Adjustment [Member] | Postretirement Benefits [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Tax (benefit) expense	(0.0)	(0.0)	(0.0)
Amounts reclassified from Accumulated other comprehensive income (loss)	$ (0.2)	$ (0.1)	$ 0.1