Debt Summary of Loss on Extinguishment of Debt and Debt Issuance Costs) (Details) - USD ($) $ in Millions	12 Months Ended
Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016
Debt Instrument [Line Items]
Loss on extinguishment of debt	$ 35.7	$ 40.4	$ 0.0
New Debt Issuance Costs
Debt Instrument [Line Items]
Loss on extinguishment of debt	11.0	20.6
Capitalized & Amortized Over Life of New Issuances	11.6	115.0
Total	22.6	135.6
Previously Incurred Debt Issuance Costs Or Unamortized Discount
Debt Instrument [Line Items]
Loss on extinguishment of debt	$ 24.7	$ 19.8

Amount of amortization expense attributable to debt issuance costs.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28555-108399

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3602-108585

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03(8))
-URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 225
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-04(8))
-URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration