Summary of Available for Sale (Details) - USD ($) $ in Millions	Mar. 31, 2018	Mar. 31, 2017
Summary of available-for-sale securities [Line Items]
Available-for-sale, adjusted cost	$ 1,293.2	$ 501.0
Available-for-sale, gross unrealized gains	2.1	1.0
Available-for-sale, gross unrealized losses	0.0	(0.4)
Available-for-sale, estimated fair value	1,295.3	501.6
Government agency bonds
Summary of available-for-sale securities [Line Items]
Available-for-sale, adjusted cost	723.2	227.1
Available-for-sale, gross unrealized gains	0.0	0.0
Available-for-sale, gross unrealized losses	0.0	(0.2)
Available-for-sale, estimated fair value	723.2	226.9
Municipal bonds - tax exempt
Summary of available-for-sale securities [Line Items]
Available-for-sale, adjusted cost		55.3
Available-for-sale, gross unrealized gains		0.0
Available-for-sale, gross unrealized losses		0.0
Available-for-sale, estimated fair value		55.3
Municipal bonds - taxable
Summary of available-for-sale securities [Line Items]
Available-for-sale, adjusted cost	14.9	10.0
Available-for-sale, gross unrealized gains	0.0	0.0
Available-for-sale, gross unrealized losses	0.0	0.0
Available-for-sale, estimated fair value	14.9	10.0
Time deposits
Summary of available-for-sale securities [Line Items]
Available-for-sale, adjusted cost	11.5
Available-for-sale, gross unrealized gains	0.0
Available-for-sale, gross unrealized losses	0.0
Available-for-sale, estimated fair value	11.5
Corporate bonds and debt
Summary of available-for-sale securities [Line Items]
Available-for-sale, adjusted cost	542.9	207.9
Available-for-sale, gross unrealized gains	0.0	0.1
Available-for-sale, gross unrealized losses	0.0	(0.2)
Available-for-sale, estimated fair value	542.9	207.8
Equity Securities
Summary of available-for-sale securities [Line Items]
Available-for-sale, adjusted cost	0.7	0.7
Available-for-sale, gross unrealized gains	2.1	0.9
Available-for-sale, gross unrealized losses	0.0	0.0
Available-for-sale, estimated fair value	$ 2.8	$ 1.6

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=75030427&loc=d3e22054-111558

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27232-111563

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=82896770&loc=d3e26610-111562

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
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