Accumulated Other Comprehensive Income - Reclassification out of AOCI (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Feb. 28, 2018	May 31, 2018	May 31, 2017	May 31, 2016
Postretirement Benefits [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Settlement charge		$ 0.0	$ 0.0	$ 0.0
Service cost [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amounts reclassified from Accumulated other comprehensive income (loss)		0.0	0.0
Service cost [Member] | Pension Plans [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amounts reclassified from Accumulated other comprehensive income (loss)		0.0	0.0
Service cost [Member] | Postretirement Benefits [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amounts reclassified from Accumulated other comprehensive income (loss)		0.0	0.0
Lump sum settlement charge [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amounts reclassified from Accumulated other comprehensive income (loss)		33.0	0.0
Lump sum settlement charge [Member] | Pension Plans [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amounts reclassified from Accumulated other comprehensive income (loss)		33.0	0.0
Lump sum settlement charge [Member] | Postretirement Benefits [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amounts reclassified from Accumulated other comprehensive income (loss)		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)		1.9	1.6
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)		1.8	1.3
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)		0.1	0.3
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		0.0	0.0	0.1
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		(55.0)	0.0	0.0
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		(2.1)	(1.7)	(1.7)
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		(0.1)	(0.4)	(2.8)
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		22.3	0.4	0.3
Amounts reclassified from Accumulated other comprehensive income (loss)		34.8	1.3	1.4
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		0.0	0.1	1.1
Amounts reclassified from Accumulated other comprehensive income (loss)		0.1	0.3	1.6
U.S. [Member] | Pension Plans [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Settlement charge	$ (57.3)	$ (57.3)	$ 0.0	$ 0.0