Significant Accounting Policies - Accumulated Other Comprehensive Loss Reclassification (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2018
Pension and postretirement plans
Other (expense) income, net									$ (3.8)	$ (1.6)	$ 4.4
Actuarial (loss) gain on pension and postretirement benefit obligations	$ (35.8)								(36.6)	0.4	3.3
Loss from discontinued operations, net of tax	0.0	$ 0.0	$ 0.0	$ 1.8	$ 0.4	$ 27.8	$ 83.7	$ 42.8	1.8	154.7	130.6
Provision for income taxes									54.1	53.4	(19.5)
Net income	28.6	48.4	56.7	46.7	47.3	25.1	(37.5)	(0.6)	180.4	34.3	76.0
Interest rate derivatives
Benefit for income taxes									54.1	53.4	(19.5)
Net income	$ 28.6	$ 48.4	$ 56.7	$ 46.7	$ 47.3	$ 25.1	$ (37.5)	$ (0.6)	180.4	34.3	76.0
Amortization of prior service credit | Reclassification out of Accumulated Other Comprehensive Income
Pension and postretirement plans
Other (expense) income, net									(0.3)	(1.5)	(1.9)
Lump sum settlement | Reclassification out of Accumulated Other Comprehensive Income
Pension and postretirement plans
Actuarial (loss) gain on pension and postretirement benefit obligations									0.8	0.6	0.0
Curtailment | Reclassification out of Accumulated Other Comprehensive Income
Pension and postretirement plans
Actuarial (loss) gain on pension and postretirement benefit obligations									0.0	0.0	(0.3)
Accumulated Defined Benefit Plans Adjustment | Reclassification out of Accumulated Other Comprehensive Income
Pension and postretirement plans
Provision for income taxes									0.2	(0.3)	(0.8)
Net income									(0.3)	0.6	1.4
Interest rate derivatives
Benefit for income taxes									0.2	(0.3)	(0.8)
Net income									(0.3)	0.6	1.4
Interest Rate Derivatives | Reclassification out of Accumulated Other Comprehensive Income | Interest Rate Contract
Pension and postretirement plans
Provision for income taxes										1.2	3.9
Net income										(4.5)	(5.8)
Interest rate derivatives
Interest expense, net										5.7	9.7
Benefit for income taxes										1.2	3.9
Net income										(4.5)	(5.8)
Foreign Currency Translation | Reclassification out of Accumulated Other Comprehensive Income
Pension and postretirement plans
Other (expense) income, net									0.0	1.8	0.0
Loss from discontinued operations, net of tax									0.0	19.7	0.0
Net income									0.0	21.5	0.0
Interest rate derivatives
Net income									$ 0.0	$ 21.5	$ 0.0