Significant Accounting Policies - Accumulated Other Comprehensive Loss Reclassification (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2017
Pension and postretirement plans
Other (expense) income, net									$ 1.2	$ (7.7)	$ (0.2)
Provision for income taxes									53.4	(19.5)	15.6
Net income (loss)	$ 47.3	$ 25.1	$ (37.5)	$ (0.6)	$ (61.9)	$ 81.6	$ 29.8	$ 26.5	34.3	76.0	74.1
Interest rate derivatives
Benefit for income taxes									53.4	(19.5)	15.6
Net income (loss)	47.3	25.1	(37.5)	(0.6)	(61.9)	81.6	29.8	26.5	34.3	76.0	74.1
Loss from discontinued operations, net of tax	$ 0.4	$ 27.8	$ 83.7	$ 42.8	$ 119.9	$ 4.3	$ 3.5	$ 2.9	154.7	130.6	17.6
Accumulated Defined Benefit Plans Adjustment, Net Prior Service | Reclassification out of Accumulated Other Comprehensive Income
Pension and postretirement plans
Other (expense) income, net									(1.5)	(1.9)	(1.9)
Accumulated Defined Benefit Plans Adjustment Attributable to Parent, Lump Sum Settlement | Reclassification out of Accumulated Other Comprehensive Income
Pension and postretirement plans
Other (expense) income, net									0.6	0.0	0.0
Accumulated Defined Benefit Plans Adjustment Attributable to Parent, Curtailment | Reclassification out of Accumulated Other Comprehensive Income
Pension and postretirement plans
Other (expense) income, net									0.0	(0.3)	(1.0)
Accumulated Defined Benefit Plans Adjustment | Reclassification out of Accumulated Other Comprehensive Income
Pension and postretirement plans
Provision for income taxes									0.3	0.8	1.1
Net income (loss)									(0.6)	(1.4)	(1.8)
Interest rate derivatives
Benefit for income taxes									0.3	0.8	1.1
Net income (loss)									(0.6)	(1.4)	(1.8)
Accumulated Net Gain (Loss) from Cash Flow Hedges Including Portion Attributable to Noncontrolling Interest | Reclassification out of Accumulated Other Comprehensive Income
Pension and postretirement plans
Net income (loss)									4.5	5.8	6.3
Interest rate derivatives
Net income (loss)									4.5	5.8	6.3
Accumulated Net Gain (Loss) from Cash Flow Hedges Including Portion Attributable to Noncontrolling Interest | Reclassification out of Accumulated Other Comprehensive Income | Interest Rate Contract
Pension and postretirement plans
Provision for income taxes									(1.2)	(3.9)	(3.9)
Interest rate derivatives
Interest expense, net									5.7	9.7	10.2
Benefit for income taxes									(1.2)	(3.9)	(3.9)
Accumulated Foreign Currency Adjustment Including Portion Attributable to Noncontrolling Interest | Reclassification out of Accumulated Other Comprehensive Income
Pension and postretirement plans
Other (expense) income, net									1.8	0.0	0.0
Interest rate derivatives
Loss from discontinued operations, net of tax									19.7	0.0	0.0
Accumulated Foreign Currency Adjustment Attributable to Parent | Reclassification out of Accumulated Other Comprehensive Income
Pension and postretirement plans
Net income (loss)									21.5	0.0	0.0
Interest rate derivatives
Net income (loss)									$ 21.5	$ 0.0	$ 0.0